Birth
Birth is the act or process of bearing or bringing forth offspring,[1] also referred to in technical contexts as parturition.
In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the fetus at a developmental stage when it is ready to feed and breathe.
In marsupials, the fetus is born at a very immature stage after a short gestation and develops further in its mother's womb pouch.
The first stage starts with a series of involuntary contractions of the muscular walls of the uterus and gradual dilation of the cervix.
The active phase of the first stage starts when the cervix is dilated more than about 4 cm in diameter and is when the contractions become stronger and regular.
[4] In stage three, which begins after the birth of the baby, further contractions expel the placenta, amniotic sac, and the remaining portion of the umbilical cord usually within a few minutes.
After birth the umbilical cord is clamped and cut, the baby starts to breathe air, and blood from the right ventricle starts to flow to the lungs for gaseous exchange and oxygenated blood returns to the left atrium, which is pumped into the left ventricle, and then pumped into the main arterial system.
[6] Large mammals, such as primates, cattle, horses, some antelopes, giraffes, hippopotamuses, rhinoceroses, elephants, seals, whales, dolphins, and porpoises, generally are pregnant with one offspring at a time, although they may have twin or multiple births on occasion.
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.
[2] In the case of whales, dolphins and porpoises, the single calf is normally born tail first which minimizes the risk of drowning.
In April 2018, approximately 8-month old elephant twins were sighted joining their mother's herd in the Tarangire National Park of Tanzania, estimated to have been born in August 2017.
The cow scrambles to her feet (if lying down at this stage), turns round and starts vigorously licking the calf.
The third and final stage of labor is the delivery of the placenta, which is usually expelled within a few hours and is often eaten by the normally herbivorous cow.
After further contractions, the sac is expelled and the bitch breaks the membranes, releasing clear fluid and exposing the puppy.
[16] Many reptiles and the vast majority of invertebrates, most fish, amphibians and all birds are oviparous, that is, they lay eggs with little or no embryonic development taking place within the mother.
Millions of eggs may be produced with no further parental involvement, in the expectation that a small number may survive to become mature individuals.
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.
[18] Ovoviviparity is a mode of reproduction in which embryos develop inside eggs that remain in the mother's body until they are ready to hatch.
[21] The Alpine salamander (Salamandra atra) and several species of Tanzanian toad in the genus Nectophrynoides are ovoviviparous, developing through the larval stage inside the mother's oviduct and eventually emerging as fully formed juveniles.
A fossil from Australia's Gogo Formation, laid down in the Devonian period, 380 million years ago, was found with an embryo inside it connected by an umbilical cord to a yolk sac.
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.
