Rare condition affecting 1-2% of pregnancies. Fetal membranes "double back" around placenta edges. Chorionic plate is smaller than basal plate
Birth is initiated by hormones causing uterine contractions to expel fetus. Human birth typically occurs 40 weeks after last menstrual period. Normal childbirth has three stages: dilation, delivery, and placenta removal. Large mammals usually give birth to precocial offspring that can move immediately. Whales, dolphins, and porpoises typically give birth tail first
Decidua is modified uterine lining that forms monthly for pregnancy. Forms maternal part of placenta and remains during pregnancy. Under progesterone influence and sheds monthly when no fertilized egg
Chorionic villi sprout from chorion to provide maternal blood contact. Umbilical arteries carry embryonic blood to villi. Blood returns to embryo through umbilical vein. Villi invade uterine decidua and absorb nutritive materials
Placenta forms in womb during pregnancy and connects to baby via umbilical cord. Provides oxygen, nutrients and removes waste from baby's blood. Attaches to uterus wall, usually top, side, front or back
Placenta develops from blastocyst's trophoblast cells during implantation. Trophoblast differentiates into syncytiotrophoblast and cytotrophoblast on day 8. Early uteroplacental circulation established by week 2. Chorionic villi form from primary to tertiary structures by week 3