The placenta is a round or oval disk, 20 × 15 cm in size and 1.5 to 2.0 cm thick. The weight, usually 20 percent of that of the fetus, is between 425 and 550 g.
On the uterine side, there are eight or more maternal cotyledons separated by fissures. The term fetal cotyledon refers to the part of the placenta that is supplied by a mainstem villus and its branches. The maternal surface is covered by a layer of decidua and fibrin, which comes away with the placenta at delivery. The fetal side is covered by membranes.
Normally, the placenta is implanted in the upper part of the uterus or fundus. Occasionally, it is placed in the lower segment, and sometimes it lies over the cervix. The latter condition is termed placenta previa and is a cause of bleeding in the third trimester.
ABNORMALITIES OF THE PLACENTA
This is an accessory lobe that is placed at some distance from the main placenta. The blood vessels that supply this lobe run over the intervening membranes and may be torn when the latter rupture or during delivery. A succenturiate lobe may be retained after birth and cause postpartum hemorrhage (PPH).
The membranes are folded back on the fetal surface and insert inward on themselves. The placenta is situated outside of the chorion.
This is a yellow nodule, 3 to 4 cm in diameter, situated on the fetal surface of the amnion. It contains vernix, fibrin, desquamated cells, and lanugo hairs. It may form a cyst. This condition is associated with oligohydramnios.
Localized infarcts are common. The clinical significance is not known, but if the condition is excessive, the functional capacity of the placenta may be reduced.
Red staining is associated with hemorrhage. Green color is caused by meconium and may be an indication of fetal hypoxia.
In monochorionic twins the placenta forms one mass, whereas in dichorionic twins the placentas may be fused or separate.
Placentas weighing more than 600 g or less than 400 g are usually associated with an abnormal pregnancy.
Delivery of the placenta occurs in two stages: (1) separation of the placenta from the wall of the uterus and into the lower uterine segment and/or the vagina and (2) actual expulsion of the placenta out of the ...