The placenta is a temporary organ that develops during gestation, acting as the vital link between the parent and the developing fetus. Its primary roles are facilitating the exchange of nutrients, oxygen, and waste products while also producing hormones necessary to sustain the pregnancy. The organ functions as a selectively permeable barrier, which is central to a healthy pregnancy.
The Placenta’s Developmental Timeline
The formation of the placenta begins shortly after the fertilized egg implants into the uterine wall, typically seven to ten days after conception. Cells from the blastocyst differentiate, with the trophoblast layer giving rise to the components that will form the placenta. The placenta continues to grow rapidly throughout the first trimester, becoming fully functional around 10 to 12 weeks of gestation. At this point, it takes over the primary role of hormone production, specifically estrogen and progesterone, which were previously managed by the corpus luteum. Throughout the pregnancy, the placenta expands in size, growing to an organ that weighs about 700 grams at full term.
Understanding Placental Transfer and Permeability
The placenta is a complex, selectively permeable membrane designed for active exchange, not a simple, impenetrable wall. The exchange occurs across the villi, where maternal blood bathes the fetal blood vessels without the two blood supplies directly mixing. Small molecules like oxygen and carbon dioxide pass easily through simple diffusion, driven by concentration gradients. Many essential nutrients, such as glucose and amino acids, require specialized protein transporters for facilitated diffusion or active transport to ensure efficient delivery to the fetus.
This selective permeability limits the passage of certain substances, offering a degree of protection to the fetus. For instance, large protein molecules and most maternal blood cells are blocked from crossing the barrier. However, the placenta’s permeability is not absolute, as many medications, viruses, and toxins, particularly small, lipid-soluble ones, can still cross and affect the developing fetus. The capacity for transfer changes throughout gestation, generally increasing later in the pregnancy to support growing fetal demands.
The Definitive End: Delivery of the Placenta
The definitive end of the placenta’s function and its permeability occurs only after the organ is expelled from the body. This event marks the third stage of labor, beginning immediately after the birth of the baby. Following the delivery of the newborn, the uterus continues to contract, causing the placenta to detach from the uterine wall. This separation physically removes the organ from the circulatory system, ceasing all exchange between parent and baby. The detached placenta is then expelled through the vagina, typically within 5 to 30 minutes after the baby’s birth.