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The placenta is an ephemeral (temporary) organ present in female placental vertebrates during gestation (pregnancy), but a placenta has evolved independently also in other animals as well, for instance scorpions and velvet worms.
The placenta receives nutrients, oxygen, antibodies and hormones from the mother's blood and passes out waste. It forms a barrier, the placental barrier, which filters out some substances which could harm the fetus. Many substances are not filtered out, however, including alcohol and some chemicals associated with smoking cigarettes. Several types of virus, such as Human Cytomegalovirus, may also cross this barrier; this often leads to various degrees of birth defects in the infant.
In addition to the transfer of gases and nutrients, the placenta also has metabolic and endocrine activity. It produces, amongst other hormones, progesterone, which is important in maintaining the pregnancy; somatomammotropin (also known as placental lactogen), which acts to increase the amount of glucose and lipids in the maternal blood; estrogen; relaxin, and human chorionic gonadotrophin HCG. This results in increased transfer of these nutrients to the fetus and is also the main cause of the increased blood sugar levels seen in pregnancy. The site of the former umbilical cord attachment in the center of the front of the abdomen is known as the umbilicus, navel, or belly-button.
When the fetus is delivered, the placenta is delivered afterwards (and for this reason is often called the afterbirth). After delivery of the fetus the umbilical cord is usually clamped and severed prior to the delivery of the placenta or may be left attached to fall off naturally which is referred to as a Lotus Birth. The placenta is delivered mainly by uterine contractions, but the umbilical cord can be gently pulled while pressure is placed on the abdomen, to speed its delivery. Usually the placenta delivers in 2-10 minutes after delivery of the newborn, but it is considered normal as long as the placenta delivers within 30 minutes. If the placenta fails to deliver in 30 minutes, manual extraction may be required, and in some cases a curettage is necessary to ensure that no remnants of the placenta remain. Risks of retained placenta include hemorrhage and infection.
In most mammalian species, the mother bites through the cord and consumes the placenta, primarily for the benefit of prostaglandin on the uterus after birth. This is known as placentophagy.
Although a predominant feature in eutherian mammals, placentae have evolved separately in snakes, as well as some invertebrates.
The major group mammals referred to as "placental mammals" belong to eutheria. In actuality marsupials (metatheria) all have placentae as well. Egg laying monotremes do not. Aside from the bandicoot,[1] most of the placenta in marsupials are not considered true-placenta, as they do not invasively implant into the uterine wall and tend to function for only a fraction of the time (see below).
The shape and exchanging surfaces of eutherian mammals varies according to family:
Marsupials are often assumed to not possess placentae, although some may [2] probably from confusion rising from the use of "placental mammal" to refer to eutherian species. All marsupials have fully functional "chorio vitelline" or "yolk sac" placentas.[3] Because marsupial young are birthed early compared to eutherian species, this placenta is not required to function as long, but is still essential for gestation of the foetus. They contrast with the large, well-developed chorio-allantoic placentae of the eutheria. However, Mossman[4] describes the eutherian placenta as "an apposition or fusion of the fetal membranes to the uterine mucosa for physiological exchange". This process has been shown to occur in the tammar wallaby,[5] and is likely evident in other macropods (kangaroos).
As mentioned, the bandicoot is considered to be a "placental marsupial" because it has a chorio-allantoic structure similar to eutherian species as well as the chorio vitelline structure associated with marsupials.[2]
<gallery> Image:Gray30.png|Fetus of about eight weeks, enclosed in the amnion. Magnified a little over two diameters. </gallery>
The placata connects the mother and the ddeveloping embryo but the mothers and the embryos blood vessels remain seperpate Bold textItalic text