Birth

==Human childbirth==
{{main|Childbirth}}
{{Further |Adaptation to extrauterine life}}
[[File:Cephalicpre.JPG|right|upright|thumb|An illustration of normal [[Cephalic presentation|head-first presentation]] by the obstetrician [[William Smellie (obstetrician)|William Smellie]] from about 1792. The [[Rupture of membranes|membranes have ruptured]] and the cervix is fully dilated.]]
Humans usually produce a single offspring at a time. The mother's body is prepared for birth by [[hormone]]s produced by the [[pituitary gland]], the [[ovary]] and the [[placenta]].<ref name=Dorit>{{cite book |title=Zoology |last1=Dorit |first1=R. L. |last2=Walker |first2=W. F. |last3=Barnes |first3=R. D. |year=1991 |publisher=Saunders College Publishing |isbn=978-0-03-030504-7 |pages=[https://archive.org/details/zoology0000dori/page/526 526–527] |url=https://archive.org/details/zoology0000dori/page/526 }}</ref> The total [[gestation period]] from [[human fertilization|fertilization]] to birth is normally about 38 weeks (birth usually occurring 40 weeks after the last [[Menstruation|menstrual period]]). The normal process of childbirth takes several hours and has three stages. 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&nbsp;cm in diameter and is when the contractions become stronger and regular. The head (or the buttocks in a [[breech birth]]) of the baby is pushed against the [[cervix]], which gradually dilates until it is fully dilated at 10&nbsp;cm diameter.  At some time, the [[amniotic sac]] bursts and the [[amniotic fluid]] escapes (also known as [[rupture of membranes]] or breaking the water).<ref>NICE (2007). Section 1.6, ''Normal labour: first stage''</ref> In stage two, starting when the cervix is fully dilated, strong contractions of the uterus and active pushing by the mother expels the baby out through the [[Human vagina|vagina]], which during this stage of labour is called a birth canal as this passage contains a baby, and the baby is born with [[umbilical cord]] attached.<ref>NICE (2007). Section 1.7, ''Normal labour: second stage''</ref> 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.<ref>NICE (2007). Section 1.8, ''Normal labour: third stage''</ref>

Enormous changes take place in the newborn's circulation to enable [[breathing]] in air. In the uterus, the [[fetus]] is dependent on circulation of blood through the placenta for sustenance including gaseous exchange and the unborn baby's blood bypasses the lungs by flowing through the [[foramen ovale (heart)|foramen ovale]], which is a hole in the septum dividing the [[right atrium]] and [[left atrium]]. 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.  As a result of these changes, the blood pressure in the left atrium exceeds the pressure in the right atrium, and this pressure difference forces the foramen ovale to close separating the left and right sides of the heart. The [[umbilical vein]], [[umbilical arteries]], [[ductus venosus]] and [[ductus arteriosus]] are not needed for life in air and in time these vessels become ligaments (embryonic remnants).<ref>{{cite book |title=The Human Body Book |page=215 |year=2007 |isbn=978-1-8561-3007-3 |publisher=[[Dorling Kindersley]] |editor1=Houston, Rob |editor2=Lea, Maxine (art)}}</ref>