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Birth

Birth is the act or process by which an organism produces and brings forth its offspring. In placental mammals, including humans, this typically involves the expulsion of a fetus from the uterus after a full-term pregnancy of approximately 40 weeks, culminating in the delivery of the newborn and the placenta.^[1] This event, also known as childbirth or parturition, involves coordinated hormonal, muscular, and mechanical changes in the mother's body to facilitate the baby's transition from intrauterine to extrauterine life.^[2] Labor usually begins spontaneously between 37 and 42 weeks of gestation, though it can be induced medically if necessary.^[1] This article discusses birth primarily in humans, with dedicated sections on other mammals and non-mammalian animals. The process of birth is divided into three main stages. The first stage, the longest and often most variable, begins with the onset of regular uterine contractions that cause the cervix to thin (efface) and dilate up to 10 centimeters; it is subdivided into early labor (up to 6 cm dilation) and active labor (6-10 cm), potentially lasting from hours to a day or more for first-time mothers.^[3] The second stage starts with full cervical dilation and involves the active pushing phase, during which the baby descends through the birth canal and is delivered vaginally, typically lasting 20 minutes to 2 hours.^[4] Finally, the third stage entails the delivery of the placenta, which is expelled from the uterus shortly after the baby's birth, usually within 30 minutes, accompanied by the cutting of the umbilical cord.^[5] Birth can occur through vaginal delivery, the most common method for low-risk pregnancies, or cesarean section (C-section), a surgical procedure performed when vaginal birth poses risks to the mother or baby, such as in cases of fetal distress or breech presentation; C-sections accounted for 32.4% of births in the United States as of 2024.^[6] Pain management options during labor include epidural anesthesia, opioids, or non-pharmacological techniques like breathing exercises and water immersion.^[1] While most births proceed normally, complications such as preterm labor (before 37 weeks), prolonged labor, or postpartum hemorrhage can arise, necessitating medical intervention to ensure the health of both mother and infant.^[2]

Biological Foundations

Definition and Terminology

Birth is the biological process by which offspring are delivered from the reproductive tract of a parent organism following a period of gestation or internal development. This event marks the transition of the young from dependent embryonic or fetal stages to independent existence outside the parent's body, typically involving the expulsion of the offspring along with associated structures such as the placenta or eggshell remnants. In most contexts, birth encompasses both live-bearing and egg-laying mechanisms, though it is often associated specifically with viviparous species where young emerge fully formed. A fundamental distinction in reproductive biology lies between viviparity, oviparity, and ovoviviparity. Viviparity refers to the production of live young that develop inside the mother's body, nourished via a placenta or similar structure until birth, as seen in most mammals. Oviparity involves the laying of eggs that contain all necessary nutrients for embryonic development outside the parent, common in birds, reptiles, and many fish. Ovoviviparity, a hybrid form, entails retaining eggs within the mother's body until the young hatch internally and emerge live, without direct nutrient transfer, exemplified in some sharks and reptiles. These modes represent adaptive strategies for offspring survival in diverse environments. Key terminology in the study of birth includes several standardized terms with roots in Latin and Greek. Parturition denotes the act of giving birth, derived from the Latin "partus" meaning "to bring forth," and is used interchangeably with "labor" in mammalian contexts to describe the physiological process of expulsion. The gestation period is the duration from conception to birth, varying widely across species; for instance, it spans about 20 days in some rodents but extends to over 600 days in elephants, reflecting evolutionary adaptations to developmental needs. A neonate is the newborn organism immediately post-birth, from the Greek "neo" (new) and Latin "natus" (born), emphasizing the vulnerable early stage requiring immediate care. These terms are employed consistently in biological literature to facilitate cross-species comparisons. The variability in birth timing underscores the diversity of reproductive strategies. Short gestation periods, such as those under a month in certain small mammals, allow for rapid reproduction in unstable environments, while prolonged periods in large herbivores enable advanced fetal maturation for survival in open habitats. This range highlights how birth timing is tuned to ecological pressures, though the core process of delivery remains a critical reproductive milestone across taxa.

Evolutionary Origins

The transition from oviparity to viviparity in vertebrates represents a major evolutionary shift, with the earliest fossil evidence dating to the Devonian period approximately 380 million years ago. A remarkable specimen of the placoderm fish Materpiscis attenboroughi from the Gogo Formation in Australia preserves an embryo connected to the mother by an umbilical cord, indicating internal gestation and live birth.^[7] This discovery, along with embryos found in related species like Austroptyctodus gardneri, demonstrates that internal fertilization and viviparity originated early within placoderms, the dominant jawed fishes of the time, predating similar traits in modern chondrichthyans.^[7] These armored fishes, as basal gnathostomes, bridge aquatic origins to later terrestrial lineages, suggesting viviparity evolved as an adaptation for protected embryonic development in ancient aquatic environments that foreshadowed terrestrial challenges.^[8] Viviparity conferred adaptive advantages by enhancing offspring survival, particularly during environmental transitions such as the move to land in early tetrapod ancestors. In transitional species like Devonian placoderms and their descendants—early fish-amphibian forebears—live birth allowed mothers to shield embryos from aquatic predators and fluctuating conditions, reducing mortality compared to exposed eggs.^[9] This protection extended to emerging terrestrial habitats, where viviparous reproduction mitigated risks from desiccation, temperature extremes, and predation, enabling higher juvenile survival rates in variable ecosystems.^[10] For instance, maternal thermoregulation in viviparous lineages maintained optimal developmental temperatures, a benefit especially pronounced in cooler or unstable climates that oviparous species struggled to exploit.^[9] Key evolutionary milestones include the development of more sophisticated reproductive structures in later vertebrates. In mammals, placentation emerged around 160 million years ago, coinciding with the diversification of therian mammals during the Jurassic, transforming simple yolk-sac nourishment into nutrient exchange via chorioallantoic placentas for prolonged gestation.^[11] Concurrently, marsupials evolved pouch systems—folds of skin enclosing mammary glands—as an intermediate strategy, allowing underdeveloped young to complete development externally while protected, likely arising after the therian-placental split around 160 million years ago.^[11] These innovations diversified birth mechanisms across amniotes, with viviparity appearing independently in squamates and other reptiles by the Mesozoic.^[8] Natural selection played a pivotal role in refining birth timing and mechanisms to optimize the balance between parental investment and offspring viability. In viviparous species, selection favored delayed parturition to maximize maternal provisioning—such as through matrotrophy—while avoiding excessive energy costs or risks to the mother, as seen in the temporal spreading of investment over gestation.^[12] Parent-offspring conflicts influenced this evolution, with embryos extracting resources up to a point where maternal fitness benefits outweighed further investment, leading to adaptive trade-offs in gestation length and litter size across lineages.^[13] This selective pressure ensured viable, independent young at birth, enhancing overall reproductive success in diverse environments.^[12]

Human Childbirth

Physiology and Stages

Human childbirth, or parturition, involves coordinated physiological processes in the female reproductive system, primarily the uterus, cervix, and placenta, regulated by hormones such as oxytocin and prostaglandins. The uterus, a muscular organ, contracts rhythmically to expel the fetus, while the cervix dilates and effaces to allow passage through the birth canal; the placenta, attached to the uterine wall, provides nourishment during pregnancy but detaches post-delivery. Labor is initiated by rising levels of estrogen relative to progesterone, which sensitizes the myometrium to contractile stimuli, and involves prostaglandins produced in the fetal membranes and decidua that promote cervical ripening and uterine contractions. Oxytocin, released from the posterior pituitary, amplifies these contractions by binding to receptors on uterine smooth muscle, creating a positive feedback loop that intensifies labor progression.^[14] The process unfolds in three distinct stages, each characterized by specific physiological changes. The first stage, dilation, begins with the onset of regular contractions and ends with full cervical dilation to approximately 10 cm; it typically lasts 12 to 20 hours in first-time births, divided into latent (early, slower dilation) and active (faster dilation) phases, during which the cervix thins (effaces) and opens under pressure from descending amniotic fluid and fetal head. Contractions in this stage start mild and irregular, increasing in frequency, duration (from 30-45 seconds to 60-90 seconds), and intensity (from 20-30 mmHg to 50-80 mmHg pressure), driven by oxytocin pulses that rise in amplitude and frequency. The second stage, expulsion, commences at full dilation and involves the delivery of the baby, lasting 20 minutes to 1 hour in nulliparous women; here, the fetus typically descends in vertex presentation—head flexed with the occiput leading—to navigate the pelvis, aided by maternal pushing triggered by the Ferguson reflex, a neuroendocrine response where cervical and vaginal distension stimulates further oxytocin release, enhancing the urge to bear down and fetal propulsion. The third stage, placental delivery, occurs 5 to 30 minutes after birth, marked by uterine contractions that shear the placenta from the endometrium, followed by its expulsion as the uterus contracts to reduce bleeding.^[2]^[15]^[16] Throughout labor, hormonal feedback loops maintain progression: prostaglandins facilitate gap junction formation in the myometrium for synchronized contractions, while oxytocin levels double during the latent phase and continue rising, peaking in the second stage to support expulsion via the Ferguson reflex. Fetal positioning is crucial; the optimal vertex (cephalic) presentation occurs in about 95% of term births, allowing the smallest head diameter to engage the cervix first, but malpositions can contribute to dystocia—abnormally slow labor due to inadequate contractions, cephalopelvic disproportion, or inefficient fetal descent—potentially prolonging stages without resolving underlying mismatches between maternal pelvis and fetal size. These mechanisms ensure efficient energy use, with the placenta's role shifting post-expulsion to involution, where continued oxytocin and prostaglandin action promotes uterine tone.^[17]^[2]^[18]

Medical and Cultural Practices

The field of obstetrics has evolved significantly from ancient practices centered on midwifery to contemporary surgical interventions. In ancient Egypt around 1550 BCE, midwifery was a recognized practice documented in texts like the Ebers Papyrus, where female practitioners assisted in deliveries using herbal remedies and manual techniques without formal obstetric training.^[19] This tradition persisted through various civilizations until the 18th and 19th centuries, when modern obstetrics emerged with the development of anesthesia and antisepsis. The first recorded successful cesarean section in the United States, where both mother and child survived, was performed by physician Jesse Bennett in 1794 on his wife in rural Virginia, marking a pivotal advancement in surgical childbirth.^[20] Common medical interventions in human childbirth include analgesia, induction of labor, episiotomy, and electronic fetal monitoring to manage risks during the stages of labor. Epidural analgesia, administered to block pain signals in the lower body, is used in approximately 70% of hospital births in the United States to alleviate labor discomfort, though it can prolong the second stage and increase the need for assisted delivery.^[21] Labor induction, often via synthetic oxytocin or prostaglandins, is employed when pregnancy exceeds 41 weeks or for medical indications, affecting about 25% of births in the United States.^[21] Episiotomy, a surgical incision to enlarge the vaginal opening, was once routine but is now recommended only selectively to prevent severe tears, with rates declining to under 15% in many settings.^[21] Continuous electronic fetal heart rate monitoring tracks the baby's well-being during contractions, used in over 80% of U.S. hospital births, though intermittent auscultation is preferred for low-risk cases to reduce unnecessary interventions.^[21] Cesarean sections, the most common major intervention, occur in 21% of global births as of 2021, with rates projected to increase to 28.5% by 2030, exceeding the World Health Organization's recommended 10-15% threshold in many regions due to factors like maternal request and provider practices.^[22]^[23] Cultural practices surrounding birth vary widely, influencing rituals and settings across societies. In some Indigenous communities, such as certain Native American tribes, water births are traditionally practiced for their perceived soothing effects, with immersion in warm water facilitating delivery in natural environments supported by community elders.^[24] Islamic postpartum care emphasizes rituals like the Aqiqah, a sacrificial offering on the seventh day after birth to express gratitude, alongside male circumcision and naming ceremonies, which reinforce communal and spiritual bonds during the nifas (postpartum bleeding) period lasting up to 40 days.^[25] Modern trends show a resurgence in home births, which increased 77% in the U.S. from 2004 to 2017 and further rose post-pandemic, comprising nearly 2% of total births as of 2023, as individuals seek personalized, low-intervention experiences with certified midwives over hospital settings.^[26] Ethical considerations in obstetrics prioritize informed consent, minimization of unnecessary interventions, and addressing disparities in maternal outcomes. Respect for patient autonomy requires providers to discuss risks, benefits, and alternatives for interventions like inductions or cesareans, ensuring decisions align with the woman's values rather than routine protocols.^[27] Efforts to reduce overmedicalization focus on evidence-based practices, such as avoiding elective episiotomies, to prevent harm from procedures lacking clear benefits.^[21] Maternal mortality disparities persist, with rates in low-income countries reaching 1 in 66 lifetime risk compared to 1 in 8,000 in high-income nations, largely due to limited access to emergency obstetric care in regions like sub-Saharan Africa.^[28]

Birth in Mammals

Placental Mammals

In placental mammals, the placenta functions as a vital organ that facilitates the exchange of oxygen, nutrients, and waste products between the maternal bloodstream and the developing fetus, allowing for prolonged internal gestation periods that support advanced fetal maturation. This structure enables gestation lengths that vary widely across species, from approximately 19 to 21 days in house mice to about 22 months in African elephants, reflecting adaptations to body size, metabolic rates, and environmental demands. Unlike other mammals, this placental nourishment permits the fetus to remain fully enclosed within the uterus until birth, minimizing external risks during development. The birth process, or parturition, in placental mammals typically unfolds in three stages akin to human childbirth: an initial phase of uterine contractions and cervical dilation lasting hours to a day, followed by active expulsion of the offspring, and finally the delivery of the placenta. In species producing litters, such as cats, adaptations include rapid successive births with intervals of 10 to 60 minutes between kittens, and placentas that may be expelled individually or in groups to expedite the process. These stages are hormonally regulated by progesterone withdrawal and oxytocin surges, ensuring coordinated labor tailored to species-specific needs. Representative examples highlight these variations. In cattle, calving follows a gestation of roughly 283 days, often requiring human assistance in farming contexts to manage dystocia, which affects 13 to 15% of births and can lead to complications if not addressed promptly. Dogs undergo whelping after a 63-day gestation, marked by the appearance of green discharge from placental separation as an early labor signal, with litters of 1 to 12 puppies delivered over several hours. Horses experience foaling after approximately 340 days, typically while standing, allowing the foal to drop to the ground and stand within minutes of birth to begin nursing. Placental mammals display a spectrum of offspring maturity at birth, influencing maternal investment. Precocial young, as in deer and horses, are born with eyes open, furred, and capable of standing and following the mother almost immediately, enhancing early predator evasion. In contrast, altricial young like those of rabbits emerge blind, hairless, and immobile, demanding prolonged nest confinement and intensive care. Following delivery, maternal behaviors universally include licking the offspring to remove amniotic fluids and stimulate respiration, consuming the placenta to reduce scent cues for predators, and initiating nursing to provide colostrum-rich milk, all of which foster bonding and survival in diverse ecological niches.

Marsupials and Monotremes

Marsupials and monotremes represent distinct reproductive strategies among mammals, diverging from the prolonged internal gestation typical of placental mammals by emphasizing external development post-birth or hatching. In marsupials, gestation is brief, resulting in the birth of highly underdeveloped young that complete much of their growth in a maternal pouch, while monotremes retain the ancestral trait of oviparity, laying eggs that hatch into altricial offspring nourished by milk. These approaches reflect evolutionary adaptations that prioritize mobility and energy efficiency in diverse habitats, such as arboreal or burrowing lifestyles.^[29] Marsupials exhibit short gestation periods, often lasting 12 to 36 days depending on the species, after which the tiny, embryonic joey emerges and instinctively crawls from the birth canal to the mother's pouch to attach to a teat for continued nourishment and protection. For instance, in kangaroos, gestation spans approximately 30 to 36 days, yielding a joey about the size of a jellybean that relies on the pouch for up to several months of development. Opossums demonstrate polyestrous breeding, with multiple litters per year and a gestation of only 12 to 14 days, producing 1 to 13 kits per litter that migrate to the pouch shortly after birth. Koalas, in contrast, typically produce singleton offspring following a 35-day gestation, with the joey remaining in the pouch for around six months. This externalized development is facilitated by incomplete placentation, where nutrient transfer via a yolk-sac placenta is limited, supporting only early embryonic stages before birth.^[30]^[31]^[32]^[33] Monotremes, the egg-laying mammals including the platypus and echidnas, further diverge by producing leathery-shelled eggs after a brief internal gestation, with embryos relying on yolk for initial nutrition. In the platypus, gestation lasts about 21 days before 1 to 3 eggs are laid and incubated for roughly 10 days in a burrow, after which the hatchlings lap milk from specialized mammary gland patches lacking nipples. Echidnas carry a single egg internally for 16 to 28 days before laying it into a temporary pouch for an additional 10 days of incubation, with the puggle emerging to feed on milk secretions for several months. The eggs' yolk provides essential lipids and proteins during this phase, a reptilian holdover that underscores monotremes' basal position in mammalian evolution.^[34]^[35]^[36] These birth strategies in marsupials and monotremes retain reptilian-like traits, such as minimal internal embryonic support and external nurturing, which enhance maternal mobility in challenging environments compared to the extended in utero development of placental mammals. This evolutionary retention likely arose from the therian-mammal ancestor's adaptations around 160-170 million years ago, balancing endothermy and parental investment with ancestral oviparous elements.^[37]

Birth in Non-Mammalian Animals

Birds and Reptiles

In birds, reproduction occurs via internal fertilization, during which sperm is transferred from the male to the female through cloacal contact, enabling the development of embryos within hard-shelled eggs laid by the female.^[38] These calcified shells, formed in the oviduct, protect the embryo and provide a gaseous exchange barrier, with the embryo nourished by yolk reserves.^[39] Incubation periods vary by species but typically last 18–30 days; for example, the domestic chicken (Gallus gallus domesticus) requires approximately 21 days of constant warmth around 37.5–38°C for embryonic development to reach hatching.^[40] Parental brooding is a key adaptation, where one or both parents use body heat to maintain optimal temperatures, often turning eggs to ensure even development and prevent embryo adhesion to the shell. A notable variation among birds is seen in megapodes (family Megapodiidae), which eschew direct parental brooding in favor of environmental heat sources for incubation.^[41] These precocial birds, including species like the malleefowl (Leipoa ocellata), construct large mound nests from decaying vegetation or use geothermal soils, where temperatures are regulated by the parents through mound adjustments rather than body contact, allowing embryos to develop over extended periods of 49–90 days without ongoing attendance.^[42] Hatchlings emerge fully feathered and independent, capable of immediate locomotion and foraging, highlighting an evolutionary divergence from typical avian parental investment.^[43] The hatching process in birds begins internally as the embryo absorbs remaining yolk and fluid, then uses an temporary egg tooth on its beak to pip through the shell, aided by enzymes secreted to weaken the inner chorioallantoic membrane and facilitate emergence.^[44] In precocial species like ducks (Anas spp.), young hatch with eyes open, downy feathers, and advanced motor skills, enabling them to leave the nest and follow parents within hours, often swimming and feeding independently shortly after.^[45] Eggs often exhibit camouflage adaptations, such as speckled or mottled patterns that blend with nest substrates like ground litter or bark, reducing predation risk during the unattended periods before or after laying.^[46] Reptiles, in contrast, predominantly employ oviparity with leathery, parchment-like eggs that lack the rigid calcification of avian shells, allowing flexibility and moisture retention in terrestrial environments.^[47] Fertilization is internal, similar to birds, via hemipenial insertion in males, but many species bury eggs in soil or sand for protection and natural incubation.^[48] However, viviparity has evolved independently in about 20% of squamate reptiles, including boas (Boa constrictor), where embryos develop within the mother's body, nourished via a simple placenta, leading to live birth of fully formed young after 5–7 months of gestation.^[49] In oviparous reptiles like sea turtles (family Cheloniidae), females undertake mass nesting events, known as arribadas in species such as the olive ridley (Lepidochelys olivacea), where thousands synchronize to lay clutches of 50–150 leathery eggs on beaches, minimizing individual predation risk through sheer numbers.^[50] Incubation lasts 45–75 days, depending on sand temperature and depth, with embryos relying on geothermal and solar heat; for loggerhead turtles (Caretta caretta), the average is around 60 days.^[51] Hatching involves enzymatic softening of the internal membranes and mechanical breaking of the shell using a caruncle (temporary egg tooth), resulting in precocial hatchlings that emerge en masse at night and instinctively orient toward the sea.^[44] Some reptiles exhibit unique reproductive strategies, such as parthenogenesis in certain whiptail lizards (Aspidoscelis spp., formerly Cnemidophorus), all-female species where unfertilized eggs develop into genetic clones via automixis, producing 2–4 young per clutch after 40–60 days of incubation in buried nests.^[52] Adaptations like temperature-dependent sex determination (TSD) are prevalent in many reptiles, including turtles and crocodilians, where pivotal incubation temperatures (e.g., 28–32°C) during the middle third of development dictate offspring sex; in many turtles, cooler conditions yield males and warmer ones females, whereas in crocodilians, intermediate temperatures typically produce males and more extreme conditions produce females—providing a bet-hedging mechanism against environmental variability.^[53]^[54] Egg camouflage in reptiles often involves translucent or earth-toned shells that match burrow substrates, though burial provides primary protection, with some species adding decoy nests to deter predators.^[55]

Fish, Amphibians, and Invertebrates

In fish, the predominant reproductive mode is oviparity, in which females release large numbers of eggs that are externally fertilized and develop outside the body, a strategy seen in most bony fish species such as trout and tuna.^[56] This approach maximizes offspring quantity but exposes eggs to high predation and environmental risks. Ovoviviparity, where fertilized eggs develop and hatch internally without maternal nourishment, occurs in various chondrichthyans like some sharks and skates, providing embryonic protection within the oviduct.^[57] Viviparity, characterized by live birth and direct maternal nutrient transfer to embryos, has arisen independently at least 22 times across fish lineages and is exemplified by poeciliids such as the guppy (Poecilia reticulata), where a placental-like structure supports development, improving offspring viability in unstable habitats.^[58] Amphibians largely reproduce via oviparity, laying eggs in aquatic or humid settings where external fertilization typically occurs, as in most frogs (Anura) and salamanders (Urodela) that deposit clutches in water to facilitate larval gill-breathing development.^[59] This ancestral mode suits their biphasic life cycle but limits distribution in arid regions. Viviparity, though uncommon (affecting fewer than 1% of species), has evolved at least eight times, often involving internal retention and nourishment; for example, many caecilians (Gymnophiona) are viviparous, with embryos receiving glandular secretions from the oviduct, while the alpine salamander (Salamandra atra) gives birth to fully formed young after extended gestation.^[59]^[58] Reproduction in invertebrates varies widely across phyla, with oviparity prevailing in groups like arthropods and mollusks, where females lay eggs that hatch into dispersive larvae, as observed in insects such as the monarch butterfly (Danaus plexippus) depositing eggs on milkweed.^[60] This strategy supports high fecundity and adaptation to diverse environments. Viviparity, rarer but convergent in several lineages, involves internal embryonic development and live birth; all scorpions (Scorpiones) are viviparous, with mothers nourishing offspring via a specialized connection before releasing them on their back.^[61] The tsetse fly (Glossina spp.) exemplifies adenotrophic viviparity, retaining one larva per cycle and provisioning it with uterine "milk" proteins until it pupates externally post-larviposition.^[62] Similar modes appear in some cockroaches and ovoviviparous sea stars like Cryptasterina hystera, balancing protection against metabolic demands.^[63]

References

[1]
Childbirth | Stages of Labor | Effacement - MedlinePlus
Jun 16, 2025 · What is childbirth? Childbirth is the process of giving birth to a baby. It includes labor and delivery of the fetus and the placenta.
[2]
Normal Labor: Physiology, Evaluation, and Management - NCBI - NIH
Feb 15, 2025 · Human labor is divided into 3 stages. The 1st stage commences with the onset of labor and ends when the cervix is fully dilated to 10 cm.
[3]
What are the stages of labor? | NICHD
Sep 1, 2017 · Stage 1. The first stage of labor happens in two phases: early labor and active labor. Typically, it is the longest stage of the process. ...
[4]
Stages of Labor: What To Expect During Labor & Delivery
In general, the three stages of labor are: Stage 1: Labor (contractions) Stage 2: Pushing and birth.Missing: reliable | Show results with:reliable
[5]
Labor | Johns Hopkins Medicine
Labor is typically divided into three stages: dilation and effacement of the cervix, pushing and the delivery of the placenta. Pain management options during ...Missing: reliable | Show results with:reliable
[6]
Labor and birth - Office on Women's Health
Sep 26, 2025 · You'll experience the amazing process of childbirth! Find out how to spot the signs of labor, and get the facts on pain management, cesarean delivery, and more.
[7]
Live birth in the Devonian period - Nature
### Key Findings on Fossil Evidence of Viviparity in Devonian Placoderms
[8]
What fossils can tell us about the evolution of viviparity and ...
It was from the Devonian period and takes the history of vertebrate placentation back to 380 million years ago. This and later fossil evidence for viviparity ...Missing: origins | Show results with:origins
[9]
Viviparity - an overview | ScienceDirect Topics
Among its benefits are that a pregnant female can protect her eggs from predators and environmental sources of mortality.
[10]
Viviparity imparts a macroevolutionary signature of ecological ...
Jun 11, 2024 · Viviparity is thought to enhance access to cold environments: whereas the species richness of oviparous species decreases with environmental ...
[11]
Genomic evolution of the placenta using co-option and duplication ...
The invention of the placenta facilitated the evolution of mammals. How the placenta evolved from the simple structure observed in birds and reptiles into the ...
[12]
The marsupial pouch: implications for reproductive success and ...
Nov 7, 2012 · The evolution of this reproductive strategy provides a clever solution to the uncertain and often adverse conditions encountered by many species ...
[13]
The evolution of pregnancy - ScienceDirect.com
Viviparity is a specialised form of intra-species parasitism which biases parental investment towards fertilised eggs, temporally spreads that investment, and ...
[14]
(PDF) Oviparity or viviparity? That is the question… - ResearchGate
The parent-offspring conflict over the degree of parental investment may be the main selective factor in the evolution of reproduction.
[15]
Endocrinology of parturition - PMC - PubMed Central - NIH
Oxytocin stimulates the release of PGE2 and prostaglandin F2α in fetal membranes by activation of phospholipase C. The prostaglandins stimulate uterine ...
[16]
Stages of labor and birth: Baby, it's time! - Mayo Clinic
early labor and active labor.Missing: reliable | Show results with:reliable
[17]
The Role of Oxytocin and the Effect of Stress During Childbirth
Oct 27, 2021 · During human parturition, the measurable amount of blood plasma oxytocin increases: it doubles during the latent phase of dilatation and ...
[18]
Executive Summary of Hormonal Physiology of Childbearing
Prelabor increases in uterine oxytocin receptors (human studies) and oxytocin receptors in brain and mammary glands (animal studies) maximize these effects.
[19]
The physiology and pharmacology of oxytocin in labor ... - PubMed
Oxytocin is released in pulses with increasing frequency and amplitude in the first and second stages of labor, with a few pulses released in the third stage of ...Missing: human | Show results with:human
[20]
Childbirth in ancient Egypt - PubMed
Care at the time of childbirth was however virtually non-existent. There were no trained obstetricians or midwives but a galaxy of gods were at hand.
[21]
Jesse Bennett (1769–1842) | Embryo Project Encyclopedia
Dec 10, 2018 · Bennett became one of the first physicians in the US to successfully complete a cesarean section after which both the woman and infant survived.Missing: 1820s | Show results with:1820s
[22]
Approaches to Limit Intervention During Labor and Birth - ACOG
Routine Amniotomy Amniotomy is a common intervention in labor and may be used to facilitate fetal or intrauterine pressure monitoring. Amniotomy also may be ...
[23]
Caesarean section rates continue to rise, amid growing inequalities ...
Jun 16, 2021 · Caesarean section use continues to rise globally, now accounting for more than 1 in 5 (21%) of all childbirths. This number is set to continue increasing over ...
[24]
The Influence of Cultural Practices on Natural Childbirth Methods
May 19, 2025 · In contrast, certain Indigenous tribes in the Americas have long practiced water births, believing that the water provides comfort and ease ...
[25]
Customs and Desirable after Childbirth, in Islam
Islamic customs after childbirth include Adhan, male circumcision, Aqiqah, shaving hair, congratulation, Walīmah, Muslim baby names, and breastfeeding.
[26]
Trends and State Variations in Out-of-Hospital Births in the United ...
Home births increased by 77%, from 0.56% of births in 2004 to 0.99% in 2017, while birth center births more than doubled, from 0.23% of births in 2004 to 0.52% ...Missing: modern | Show results with:modern
[27]
Ethical Decision Making in Obstetrics and Gynecology* | ACOG
Respect for autonomy provides a strong moral foundation for informed consent, in which a patient, adequately informed about her medical condition and the ...
[28]
Maternal mortality rates and statistics - UNICEF DATA
In 2023, the lifetime risk of maternal death in low-income countries as a whole was 1 in 66, compared to 1 in approximately 8000 in high-income countries.
[29]
Mammalian Structure and Function: Reproduction
Marsupial-Placental Dichotomy. I. Why do marsupials and placentals differ in their mode of reproduction? Marsupials, Placentals. very short gestation, usually ...
[30]
[PDF] Life Cycle Of A Kangaroo
- The gestation period for a kangaroo is relatively short, lasting about 30 to 36 days. -. After this period, the tiny, underdeveloped joey is born. Birth and ...
[31]
[PDF] Opossums - Washington Department of Fish and Wildlife
The breeding season begins as early as January and may continue to mid-November. • Being marsupials, opossums give birth to undeveloped young. Only 12 days ...<|separator|>
[32]
Studies of the oestrous cycle, oestrus and pregnancy in the koala ...
After mating of the 12 koalas, six females gave birth with a mean (+/- SEM) gestation of 34.8 +/- 0.3 days, whereas the remaining six non-parturient females ...
[33]
Marsupialia | Profiles RNS
In contrast to Eutheria (placentals), marsupials have an incomplete PLACENTA. ... Marsupial; Metatheria. Below are MeSH descriptors whose meaning is more ...
[34]
Observations on the reproductive morphology of the female short ...
In a captive colony of echidnas in which mating and egg deposition was closely monitored, the gestation period was found to be 16–17 days followed by a 10 day ...
[35]
Monotreme Lactation Protein Is Highly Expressed ... - PubMed Central
Monotremes lay eggs following a short gestation period and after an even briefer incubation period, altricial hatchlings are nourished over a long lactation ...
[36]
Loss of Egg Yolk Genes in Mammals and the Origin of Lactation and ...
Mar 18, 2008 · Egg yolk contains the nutrients required for the development of the nonmammalian vertebrate embryo. These nutrients derive by and large from a ...
[37]
[PDF] The Interrelationships of Placental Mammals and the Limits of ...
Dec 23, 2015 · Our Bayesian molecular clock analysis estimates that marsupials diverged from placentals 157–170 Ma, crown Placentalia diverged 86–100 Ma, and ...
[38]
The Avian Embryo | Mississippi State University Extension Service
A sperm cell must penetrate the thin vitelline membrane and reach the female cell to complete fertilization. The vitelline membrane thickens as the rest of the ...
[39]
[PDF] Avian Embryo - Mississippi State University Poultry Science
Incubation procedures show you the effects of heat, moisture, and ventilation upon the development of the chick embryo. You also learn to hatch other fowl such ...
[40]
[PDF] Embryology: Experiments in Poultry Science - Florida 4-H
Most eggs sold in grocery stores are not fertile and cannot be used for incubation. Fertile eggs can usually be obtained from hatcheries or poultry breeding ...
[41]
Megapodes: Recent ideas on origins, adaptations and reproduction
The megapodes have evolved a markedly different method of incubation. By making use of environmental heat, adult megapodes are emancipated from most of the ...
[42]
Energetics of Embryonic Development in the Megapode Birds ...
Their incubation periods of 62 days (MF) and 49 days (BT) are unusually long, and their hatchlings are among the most precocial of any birds. Metabolic rates of ...
[43]
(PDF) Incubator birds: Biogeographical origins and evolution of ...
Aug 7, 2025 · The evolutionary path towards novel incubation behaviours has led to ecological and physiological adaptations unique to megapodes. Here, we ...
[44]
[PDF] Effect of incubation temperature variation and ... - Auburn University
the hatching process. However, their development ... Finally, it is known that in large-scale chicken egg incubation, most of the ... abundance of the enzymes ...
[45]
Precocial versus Altricial Development – Maine Birds - Web – A Colby
Aug 6, 2009 · Some precocial chicks can feed themselves soon after hatching. A Lesser Scaup duckling can swim, dive and catch fish only three days after ...
[46]
[PDF] Imperfectly camouflaged avian eggs: artefact or adaptation?
Brown eggs probably evolved as an adaptation for camouflage, first appearing some 30–100 million years ago when birds diversified their nesting habits. Among ...
[47]
Reptiles - OpenEd CUNY
They usually reproduce sexually with internal fertilization. Some species display ovoviviparity, with the eggs remaining in the mother's body until they are ...
[48]
[PDF] Zoology: Understanding the Animal World
◇ Unlike bird eggs, which are always hard-shelled, non-avian reptiles' eggs may be hard-shelled or soft-shelled and leathery. Hard, mineralized shells ...
[49]
[PDF] The Distribution and Evolution of Viviparity in Reptiles
Dec 12, 1977 · captive boa produced two normal young as well as eggs with. "leathery shelled membranes" which enclosed young of approxi- mately the same ...
[50]
Sea Turtles - Cape Hatteras National Seashore (U.S. National Park ...
Sep 29, 2025 · Eggs have an incubation period of 49-76 days. The Cape Hatteras National Seashore is near the edge of the northern limit of their nesting range.
[51]
[PDF] Loggerhead Sea Turtle Nesting and Hatchling Habits ... - NC Wildlife
Loggerhead sea turtles incubate for about 60 days. Hatchlings then emerge from the nests and scurry towards the water. They spend several days swimming offshore ...Missing: period | Show results with:period
[52]
Cnemidophorus uniparens (Desert Grassland Whiptail)
These lizards are parthenogenic, meaning they develop in unfertilized eggs. ... The amount of eggs per clutch correlates strongly with the size of the lizard.Missing: incubation | Show results with:incubation
[53]
Temperature-Dependent Sex Determination in Reptiles
Feb 1, 2013 · Temperature-dependent sex determination (TSD), where the temperature of the embryo's environment influences its sex development, is a widespread ...Missing: birds precocial young ducks camouflage
[54]
Birds | manoa.hawaii.edu/ExploringOurFluidEarth
5.36). Marine bird eggs often have a camouflage pattern, an adaptation that reduces losses to egg-eating predators.
[55]
WFS 550 Fish Physiology - Reproduction
Most fish are oviparous, although ovoviparity and viviparity are also well represented. Parental care may occur in oviparous fishes but is quite variable.
[56]
[PDF] Fishery Science – Biology & Ecology - NET
Like ovoviviparous fish, the viviparous fish give birth to live young, however viviparous fish are fully advanced at birth allowing for a greater chance of ...
[57]
Understanding the evolution of viviparity using intraspecific variation ...
14 abr 2025 · Here we explore the biology and evolution of transitional forms of pregnancy that are midway between the ancestral state of oviparity (egg‐laying) and the ...
[58]
[PDF] Evolution of oviductal gestation in amphibians - Integrative Biology |
In anurans and urodeles obligate vivi- parity is very rare (less than 1% of species); a few additional species retain the developing young, but nutrition is ...
[59]
Section 3: Sexual Reproduction - EdTech Books
Some invertebrates are gonochoristic (having separate sexes), while others are hermaphroditic (possessing both male and female reproductive organs). Chromosomal ...
[60]
Striped Bark Scorpion - Field Guide to Common Texas Insects
All scorpions are born live (viviparous), and embryos are nourished in the female's body (in utero or via a “placental” connection). Development (gestation) is ...
[61]
Researchers Sequence Genome of Tsetse Fly | NC State News
Apr 24, 2014 · Notable differences between tsetse and other disease vectors include obligate microbial symbioses, viviparous reproduction and lactation.
[62]
Viviparity in the Sea Star Cryptasterina hystera (Asterinidae)
Cryptasterina hystera has a highly derived life history with intragonadal development and juveniles that emerge from the parent's reproductive tract.