Biology · Ch 2 — Human Reproduction
Pregnancy and Placenta
Pregnancy and Placenta
Once the blastocyst has implanted, the trophoblast layer proliferates and develops numerous finger-like projections called chorionic villi, which grow outward into the surrounding endometrial tissue. As pregnancy advances, these villi become richly supplied with foetal blood vessels (connected to the foetus via the umbilical cord) and become bathed on their outer surface by pools of maternal blood within the endometrium; together, this arrangement of foetal chorionic villi interdigitating with maternal endometrial tissue and maternal blood constitutes the placenta, a specialised organ unique to pregnancy that serves as the interface between mother and foetus for the rest of gestation.
Structurally, the placenta is attached to the foetus by the umbilical cord, which carries two umbilical arteries (carrying deoxygenated foetal blood to the placenta) and one umbilical vein (returning oxygenated, nutrient-rich blood to the foetus). Crucially, maternal and foetal blood do not normally mix directly within the placenta; instead, the two blood supplies remain separated by the thin walls of the chorionic villi, across which exchange takes place by diffusion and active transport.
Functionally, the placenta performs several roles simultaneously. It is the site of nutrient supply, allowing glucose, amino acids, fatty acids, vitamins and minerals to pass from the maternal blood to the foetal circulation. It is the site of respiratory gas exchange, allowing oxygen to diffuse from maternal blood into foetal blood and carbon dioxide to diffuse in the reverse direction, functioning as the foetus's lungs before birth. It is the route of waste removal, allowing nitrogenous wastes produced by the foetus (such as urea) to pass into the maternal circulation for excretion by the mother's kidneys. And it acts as a selective, though incomplete, barrier, allowing some substances (including many drugs and some pathogens) to cross from mother to foetus while excluding others, and allowing maternal antibodies to cross and confer some passive immunity on the foetus even before birth. …
| Hormone | Principal Role During Pregnancy |
|---|---|
| Human Chorionic Gonadotropin (hCG) | Signals and maintains the corpus luteum in early pregnancy so it continues secreting progesterone; its presence in maternal blood/urine is the basis of pregnancy testing |
| Human Placental Lactogen (hPL) | Modifies maternal metabolism to divert glucose and other nutrients toward the growing foetus; contributes to preparing the mammary glands for later lactation |
| Oestrogen | Rises steadily through pregnancy; supports growth of the uterus and of the mammary gland ducts |
| Progesterone | Secreted in increasing amounts by the placenta as pregnancy advances, taking over from the corpus luteum; maintains the endometrium and keeps the uterine muscle quiescent until term |