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Birth, also known as parturition, is the act or process of bearing or bringing forth offspring. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the foetus at a developmental stage where it can feed and breathe. In some species the offspring is precocial and can move around almost immediately after birth but in others it is altricial and completely dependent on parenting. In marsupials, the foetus is born at a very immature stage after a short gestational period and develops further in its mother's pouch.
It is not only mammals that give birth. Some reptiles, amphibians, fish and invertebrates carry their developing young inside them. Some of these are ovoviviparous, with the eggs being hatched inside the mother's body, and others are viviparous, with the embryo developing inside her body, as in mammals.
The mother's body is prepared for birth by hormones produced by the pituitary gland, the ovary and the placenta. The total gestation period from fertilization to birth is about 38 weeks (birth usually occurring 40 weeks after the last menstrual period). The normal process of giving birth starts with a series of involuntary contractions of the muscular walls of the uterus. The contractions get stronger and increase in frequency and the head (or the buttocks in a breech birth) of the baby is pushed against the cervix. The cervix gradually dilates, a process that may take many hours, especially in a woman bearing her first child. At some stage, the amniotic sac bursts and the amniotic fluid escapes. When the cervix is fully dilated, further strong contractions of the uterus push the baby out through the vagina and the baby is born with umbilical cord attached. Further contractions expel the placenta, amniotic sac, and the remaining portion of the umbilical cord, usually within a few minutes.
Humans are unusual among mammals in producing a single offspring at one time. Other mammals that do this include other primates, cattle, horses, some antelopes, giraffes, hippopotamuses, rhinoceroses, elephants, seals, whales, dolphins, and porpoises although all, including humans, may have twin or multiple births on occasion. In these large animals, the birth process is similar to that of a human though in most, the offspring is precocial. This means that it is born in a more advanced state than a human baby and is able to stand, walk and run (or swim in the case of an aquatic mammal) shortly after birth. In the case of whales, dolphins and porpoises, the single calf is normally born tail first which minimises the risk of drowning. The mother encourages the newborn calf to rise to the surface to breathe.
Most smaller mammals have multiple births, producing litters of young which may number twelve or more. In these, each foetus is surrounded by its own placenta and amniotic membranes, which separate from the wall of the uterus during labour and work their way towards the vulva. In the dog, the arrival of the first puppy is preceded by the bulging dark-coloured placenta appearing at the vulva. After further contractions, the placenta is expelled with the puppy inside. The bitch breaks the membrane, chews at the umbilical cord, and usually eats the afterbirth. She licks the puppy vigorously, which stimulates it to breathe, and it is soon looking for a teat. Meanwhile the next puppy wrapped in its sac is on its way. Each sac contains enough oxygen for the puppy to survive for about six minutes after which time it is likely to die if not yet breathing.
An infant marsupial is born in a very immature state. The gestation period is usually shorter than the intervals between oestrus periods. During gestation there is no placenta but the foetus is contained in a little yellow sac and feeds on a yolk. The first sign that a birth is imminent is the mother cleaning out her pouch. When it is born, the infant is pink, blind, furless and a few centimetres long. It has nostrils in order to breathe and forelegs to cling onto its mother's hairs but its hind legs are undeveloped. It crawls through its mother's fur and makes its way into the pouch. Here it fixes onto a teat which swells inside its mouth. It stays attached to the teat for several months until it is sufficiently developed to emerge.
Birthing in cattle is typical of a larger mammal. A cow goes through three stages of labour during delivery of a calf. During Stage I, the animal seeks a quiet place away from the rest of the herd. Hormone changes cause the ligaments of her pelvis to relax as her body prepares for birth. The contractions of the uterus are not obvious externally, but the cow may be restless. She may appear agitated, alternating between standing and lying down, with her tail slightly raised and her back arched. With each contraction, the foetus is propelled toward the birth canal, causing gradual dilation of the cow's cervix. Stage I may last several hours, and officially ends when the cervix is fully dilated. Stage II officially begins with the external protrusion of the amniotic sac, or water bag, closely followed by the calf's front hooves being visible outside the vulva. The calf's feet may disappear from view between contractions, which are now full abdominal muscle contractions that increase in frequency and strength. During this second stage, the cow will usually lie down on her side to push. The calf should make normal progress through the birth canal, with the whole head projecting externally beside the forelegs. The rest of the birth is usually quite quick and the shoulders, body, and rear legs of the calf are expelled, wrapped in the amniotic membranes. The complete delivery of the calf or calves (if multiples are born) signifies the end of Stage II. The cow scrambles to her feet (if lying down at this stage), turns round and starts vigorously licking the calf. The calf takes its first few breaths and within minutes is struggling to rise to its feet. Stage III of labour is the delivery of the afterbirth, or placenta. This is usually expelled within a few hours and is often eaten by the normally herbivorous cow. The calf noses under the cow's belly searching for a teat, and is soon feeding on the colostrum produced by the mammary glands in the udder.
The vast majority of invertebrates, most fish, reptiles and amphibians and all birds are oviparous, that is, they lay eggs with little or no embryonic development taking place within the mother. In aquatic organisms, fertilisation is nearly always external with sperm and eggs being liberated into the water (an exception is sharks and rays, which have internal fertilization). Millions of eggs may be produced with no further parental involvement, in the expectation that a small number may survive to become mature individuals. Terrestrial invertebrates may also produce large numbers of eggs, a few of which may avoid predation and carry on the species. Some fish, reptiles and amphibians have adopted a different strategy and invest their effort in producing a small number of young at a more advanced stage which are more likely to survive to adulthood. Birds care for their young in the nest and provide for their needs after hatching and it is perhaps unsurprising that internal development does not occur in birds, given their need to fly.
Ovoviviparity is a mode of reproduction in which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch. Ovoviviparous animals are similar to viviparous species in that there is internal fertilization and the young are born in an advanced state, but differ in that there is no placental connection and the unborn young are nourished by egg yolk. The mother's body provides gas exchange (respiration), but that is largely necessary for oviparous animals as well. In many sharks the eggs hatch in the oviduct within the mother's body and the embryos are nourished by the egg's yolk and fluids secreted by glands in the walls of the oviduct. The Lamniforme sharks practice oophagy, where the first embryos to hatch consume the remaining eggs and sand tiger shark pups cannibalistically consume neighbouring embryos. The requiem sharks maintain a placental link to the developing young, this practice is known as viviparity. This is more analogous to mammalian gestation than to that of other fishes. In all these cases, the young are born alive and fully functional. The majority of caecilians are oviviviparous and give birth to already developed offspring. When the young have finished their yolk sacs they feed on nutrients secreted by cells lining the oviduct and even the cells themselves which they eat with specialist scraping teeth. The Alpine salamander (Salamandra atra) and several species of Tanzanian toad in the genus Nectophrynoides are oviviviparous, developing through the larval stage inside the mother's oviduct and eventually emerging as fully formed juveniles.
A more developed form of vivipary called placental viviparity is adopted by some species of scorpions and cockroaches, certain genera of sharks, snakes and velvet worms. In these, the developing embryo is nourished by some form of placental structure. The earliest known placenta was found recently in a group of extinct fishes called placoderms, which are ancestral to mammals. A fossil from Australia's Gogo Formation, laid down in the Devonian Era, 380 million years ago, was found with an embryo inside it connected by an umbilical cord to a yolk sac. The find confirmed the hypothesis that a sub-group of placoderms, called ptyctodontids, fertilized their eggs internally. Some fishes that fertilize their eggs internally also give birth to live young, as seen here. This discovery moved our knowledge of live birth back 200 million years. The fossil of another genus was found with three embryos in the same position. Placoderms are a sister group of the ancestor of all living jawed fishes (Gnathostomata), including both chondrichthyians, the sharks & rays, and Osteichthyes, the bony fishes.
Among lizards, many species of skink are viviparous. Some are ovoviviparous but others such as members of the genera Tiliqua and Corucia, give birth to live young that develop internally, deriving their nourishment from a mammal-like placenta attached to the inside of the mother's uterus. In a recently described example, an African species, Trachylepis ivensi, has developed a purely reptilian placenta directly comparable in structure and function to a mammalian placenta.
The majority of insects lay eggs but a very few give birth to offspring that are miniature versions of the adult. The aphid has a complex life cycle and during the summer months is able to multiply with great rapidity. Its reproduction is typically parthenogenetic and viviparous and females produce unfertilised eggs which they retain within their bodies. The embryos develop within their mothers' ovarioles and the offspring are clones of their mothers. Female nymphs are born which grow rapidly and soon produce more female offspring themselves. In some instances, the newborn nymphs already have developing embryos inside them.