Placenta, Extraembryonic Membranes, and Multiple Pregnancies Part 5

Parturition • Parturition is the process of childbirth, during which the fetus, placenta, and fetal membranes are expelled from the mother's reproductive tract. • It involves labor, a sequence of involuntary uterine contractions that lead to the dilation of the uterine cervix and the expulsion of the fetus and placenta. • Several hormones are involved in the initiation of labor: • The fetal hypothalamus secretes corticotropin-releasing hormone, which stimulates the anterior pituitary to release adrenocorticotropin, leading to the secretion of cortisol from the adrenal cortex. This is related to the production of estrogens. • Oxytocin, a hormone from the pituitary gland, triggers uterine contractions and stimulates the release of prostaglandins, which increase uterine contractility. • Estrogens also enhance uterine contractile activity and promote the release of oxytocin and prostaglandins. Placenta and Fetal Membranes after Birth • The placenta is usually discoid in shape, measuring 1520 cm in diameter and 2-3 cm thick, weighing about 500-600 g, or one-sixth the weight of an average fetus. • Its margins are continuous with the ruptured amniotic and chorionic sacs. • Maternal Surface • The maternal surface has a cobblestone appearance, created by cotyledons separated by grooves where placental septa used to be. • After delivery, parts of the decidua basalis remain in the uterus and are shed with bleeding. • Fetal Surface • The umbilical cord attaches to the fetal surface, and the epithelium is continuous with the amnion, which covers the surface. • The chorionic vessels branch from the umbilical cord, forming the arteriocapillary−venous system in the chorionic villi. Placental Abnormalities • Placenta accreta: Abnormal adherence of chorionic villi to the myometrium. It can lead to complications when the placenta fails to detach after birth, causing hemorrhage. • Placenta percreta: Villi penetrate the full thickness of the myometrium or through the perimetrium, also leading to complications and hemorrhage. • Placenta previa: Occurs when the placenta implants near or over the internal os of the uterus, leading to late pregnancy bleeding and requiring cesarean delivery. Umbilical Cord • The umbilical cord typically attaches near the center of the placenta but may attach in various locations, including the placental margin (battledore placenta) or the fetal membranes (velamentous insertion). • It is about 1 to 2 cm in diameter and 30 to 90 cm in length (average 55 cm). • The cord typically contains two arteries and one vein, surrounded by Wharton jelly. Amnion & Amniotic Fluid • The amnion is a tough, fluid-filled sac that surrounds the fetus, playing a crucial role in fetal growth and development. • It contains amniotic fluid, which is essential for the fetus, aiding in development, cushioning, temperature regulation, and preventing infections. • Amniotic fluid is initially secreted by the amnion and later derived from maternal tissues and fetal urine, increasing to around 700-1000 ml by the end of pregnancy. • The fluid circulates as it is swallowed by the fetus and excreted through the kidneys, while water is exchanged between maternal and fetal blood. • Amniotic fluid is composed of organic compounds (proteins, carbohydrates, fats) and inorganic salts, and its composition changes during pregnancy. Studies of the fluid, obtained via amniocentesis, can help diagnose chromosomal abnormalities like Down syndrome and neural tube defects. Amniotic fluid also allows the fetus to move freely, which aids muscular development, and helps maintain homeostasis of fluid and electrolytes. • Disorders of amniotic fluid volume include: 1. Oligohydramnios (low fluid volume), often caused by placental insufficiency, kidney malformations, or urinary tract obstructions. It can lead to birth defects, such as pulmonary hypoplasia, and umbilical cord compression. 2. Polyhydramnios (excess fluid volume), often of unknown cause but sometimes related to maternal factors or fetal conditions like esophageal atresia or central nervous system defects. • Ultrasonography is the primary tool for diagnosing these conditions. Preterm rupture of the amniotic sac can lead to premature labor and loss of infection protection for the fetus.