Placenta in the context of 3β-hydroxysteroid dehydrogenase

Childbirth, also known as labour (or labor in American English), parturition, and delivery, is the completion of pregnancy, where one or more fetuses exits the internal environment of the mother via vaginal delivery or caesarean section and becomes a newborn to the world. In 2019, there were about 140.11 million human births globally. In developed countries, most deliveries occur in hospitals, while in developing countries most are home births.

The most common childbirth method worldwide is vaginal delivery. It involves four stages of labour: the shortening and opening of the cervix during the first stage, descent and birth of the baby during the second, the delivery of the placenta during the third, and the recovery of the mother and infant during the fourth stage, which is referred to as the postpartum. The first stage is characterised by abdominal partying or also back pain in the case of back labour, that typically lasts half a minute and occurs every 10 to 30 minutes. Contractions gradually become stronger and closer together. Since the pain of childbirth correlates with contractions, the pain becomes more frequent and strong as the labour progresses. The second stage ends when the infant is fully expelled. The third stage is the delivery of the placenta. The fourth stage of labour involves the recovery of the mother, delayed clamping of the umbilical cord, and monitoring of the neonate. All major health organisations advise that immediately after giving birth, regardless of the delivery method, that the infant be placed on the mother's chest (termed skin-to-skin contact), and to delay any other routine procedures for at least one to two hours or until the baby has had its first breastfeeding.

Among animals which produce eggs, the yolk (/ˈjoʊk/; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example because they are laid in situations where the food supply is sufficient (such as in the body of the host of a parasitoid) or because the embryo develops in the parent's body, which supplies the food, usually through a placenta. Reproductive systems in which the mother's body supplies the embryo directly are said to be matrotrophic; those in which the embryo is supplied by yolk are said to be lecithotrophic. In many species, such as all birds, and most reptiles and insects, the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. In many other animals, especially very small species such as some fish and invertebrates, the yolk material is not in a special organ, but inside the egg cell.

As stored food, yolks are often rich in vitamins, minerals, lipids and proteins. The proteins function partly as food in their own right, and partly in regulating the storage and supply of the other nutrients. For example, in some species the amount of yolk in an egg cell affects the developmental processes that follow fertilization.

An artery (from Greek ἀρτηρία (artēríā)) is a blood vessel in humans and most other animals that takes oxygenated blood away from the heart in the systemic circulation to one or more parts of the body. Exceptions that carry deoxygenated blood are the pulmonary arteries in the pulmonary circulation that carry blood to the lungs for oxygenation, and the umbilical arteries in the fetal circulation that carry deoxygenated blood to the placenta. It consists of a multi-layered artery wall wrapped into a tube-shaped channel.

Arteries contrast with veins, which carry deoxygenated blood back towards the heart; or in the pulmonary and fetal circulations carry oxygenated blood to the lungs and fetus respectively.

Placental mammals (infraclass Placentalia /plæsənˈteɪliə/) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that the fetus is carried in the uterus of its mother to a relatively late stage of development. The name is something of a misnomer, considering that marsupials also nourish their fetuses via a placenta, though for a relatively briefer period, giving birth to less-developed young, which are then nurtured for a period inside the mother's pouch. Placentalia represents the only living group within Eutheria, which contains all mammals that are more closely related to placentals than they are to marsupials.

In human reproduction, a live birth occurs when a fetus exits a pregnant female showing any definite sign of life, such as voluntary movement, heartbeat or pulsation of the umbilical cord, for however brief a time, and regardless of whether the umbilical cord or placenta are intact. After the fetus leaves the womb it is called a neonate. It is a "live birth" whether the birth is vaginal or by caesarean section, and whether the neonate is ultimately viable.

The definition of the term "live birth" was created by the World Health Organization in 1950, and is chiefly used for public health and statistical purposes. However, the term "live birth" was in common use long before 1950.

Stillbirth is typically defined as the death of a fetus at or after 20 or 28 weeks of pregnancy, depending on the source. It results in a baby born without signs of life. A stillbirth can often result in the feeling of guilt or grief in the mother. The term is in contrast to miscarriage, which is an early pregnancy loss, and sudden infant death syndrome, where the baby dies a short time after being born alive.

Often the cause is unknown. Causes may include pregnancy complications such as pre-eclampsia and birth complications, problems with the placenta or umbilical cord, birth defects, infections such as malaria and syphilis, and poor health in the mother. Risk factors include a mother's age over 35, smoking, drug use, use of assisted reproductive technology, and first pregnancy. Stillbirth may be suspected when no fetal movement is felt. Confirmation is by ultrasound.

Caesarean section, also known as C-section, cesarean, or caesarean delivery, is the surgical procedure by which one or more babies are delivered through an incision in the mother's abdomen. It is often performed because vaginal delivery would put the mother or child at risk (of paralysis or even death). Reasons for the operation include, but are not limited to, obstructed labor, twin pregnancy, high blood pressure in the mother, breech birth, shoulder presentation, and problems with the placenta or umbilical cord. A caesarean delivery may be performed based upon the shape of the mother's pelvis or history of a previous C-section. A trial of vaginal birth after C-section may be possible. The World Health Organization recommends that caesarean section be performed only when medically necessary.

A C-section typically takes between 45 minutes to an hour to complete. It may be done with a spinal block, where the woman is awake, or under general anesthesia. A urinary catheter is used to drain the bladder, and the skin of the abdomen is then cleaned with an antiseptic. An incision of about 15 cm (5.9 in) is then typically made through the mother's lower abdomen. The uterus is then opened with a second incision and the baby delivered. The incisions are then stitched closed. A woman can typically begin breastfeeding as soon as she is out of the operating room and awake. Often, several days are required in the hospital to recover sufficiently to return home.

In placental mammals, the umbilical cord (also called the navel string, birth cord or funiculus umbilicalis) is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord is physiologically and genetically part of the fetus and (in humans) normally contains two arteries (the umbilical arteries) and one vein (the umbilical vein), buried within Wharton's jelly. The umbilical vein supplies the fetus with oxygenated, nutrient-rich blood from the placenta. Conversely, the fetal heart pumps low-oxygen, nutrient-depleted blood through the umbilical arteries back to the placenta.

Colostrum (from Latin, of unknown origin), also known as foremilk, is the first form of milk produced by the mammary glands of humans and other mammals immediately following delivery of the newborn. Animal colostrum may be called beestings, the traditional word from Old English dialects. Most species will begin to generate colostrum just prior to giving birth. Colostrum contains antibodies to protect the newborn against disease and infection, and immune and growth factors and other bioactives. The bioactives found in colostrum are beneficial for a newborn's health, growth and vitality. Colostrum strengthens a baby's immune system.

At birth, the environment of the newborn mammal shifts from the sterile conditions of the mother's uterus, with a constant nutrient supply via the placenta, to the microbe-rich environment outside, with irregular oral intake of complex milk nutrients through the gastrointestinal tract. This transition puts high demands on the gastrointestinal tract of the neonate, as the gut plays an important part in both the digestive system and the immune system. Colostrum contributes significantly to initial immunological defense as well as to the growth, development, and maturation of the neonate's gastrointestinal tract by providing key nutrients and bioactive factors. Bovine colostrum powder is rich in protein and low in sugar and fat. Bovine colostrum can also be used for nonorganic failure to thrive in children and acute non-steroidal anti-inflammatory drug-induced increase in intestinal permeability in males and can boost a neonate's immunity.

Conjoined twins, popularly referred to as Siamese twins, are twins joined in utero. It is a very rare phenomenon, estimated to occur in anywhere between one in 50,000 births to one in 200,000 births, with a somewhat higher incidence in southwest Asia and Africa. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1.

Two possible explanations of the cause of conjoined twins have been proposed. The one that is generally accepted is fission, in which the fertilized egg splits partially. The other explanation, no longer believed to be accurate, is fusion, in which the fertilized egg completely separates, but stem cells (that search for similar cells) find similar stem cells on the other twin and fuse the twins together. Conjoined twins and some monozygotic, but not conjoined, twins share a single common chorion, placenta, and amniotic sac in utero.

The decidua is the modified mucosal lining of the uterus (that is, modified endometrium) that forms every month, in preparation for pregnancy. It is shed off each month when there is no fertilized egg to support. The decidua is under the influence of progesterone. Endometrial cells become highly characteristic. The decidua forms the maternal part of the placenta and remains for the duration of the pregnancy. After birth the decidua is shed together with the placenta.

The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the embryoblast which subsequently forms the embryo, and an outer layer of trophoblast cells called the trophectoderm. This layer surrounds the inner cell mass and a fluid-filled cavity or lumen known as the blastocoel. In the late blastocyst, the trophectoderm is known as the trophoblast. The trophoblast gives rise to the chorion and amnion, the two fetal membranes that surround the embryo. The placenta derives from the embryonic chorion (the portion of the chorion that develops villi). The corresponding structure in non-mammalian animals is an undifferentiated ball of cells called the blastula.

In humans, blastocyst formation begins about five days after fertilization when a fluid-filled cavity opens up in the morula, the early embryonic stage of a ball of 16 cells.The blastocyst has a diameter of about 0.1–0.2 mm and comprises 100-200 cells following 7-8 rounds of cleavage (cell division without cell growth). About seven days after fertilization, the blastocyst undergoes implantation, embedding into the endometrium of the uterine wall where it will undergo further developmental processes, including gastrulation. Embedding of the blastocyst into the endometrium requires that it hatches from the zona pellucida, the egg coat that prevents adherence to the fallopian tube as the pre-embryo makes its way to the uterus.

The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer: the basal layer contains stem cells which regenerate the functional layer. The functional layer thickens and then is shed during menstruation in humans and some other mammals, including other apes, Old World monkeys, some species of bat, the elephant shrew and the Cairo spiny mouse. In most other mammals, the endometrium is reabsorbed in the estrous cycle. During pregnancy, the glands and blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the embryo and fetus. The speculated presence of an endometrial microbiotahas been argued against.

Matrotrophy is a form of maternal care during organism development, associated with live birth (viviparity), in which the embryo of an animal or flowering plant is supplied with additional nutrition from the mother (e.g. through a placenta). This can be contrasted with lecithotrophy, in which the only source of nutrition for the embryo is yolk originally contained within its egg.