Final answer:
The placenta develops chorionic villi to increase the efficiency of nutrient and waste exchange between the mother and fetus. These villi proliferate throughout pregnancy, making the placenta more efficient. Avoidance of fetotoxic substances is necessary due to placental permeability.
Step-by-step explanation:
The placenta is a critical organ in pregnancy that facilitates the exchange of nutrients and waste between maternal and fetal blood. During gestation, the placenta's structure, particularly the chorionic villi, adapts to increase its efficiency in nutrient transfer. The trophoblast cells differentiate, leading to the formation of villi, which are projections that increase the surface area allowing for enhanced exchange. These villi will proliferate and become extensively branched, making the placental barrier thinner and more efficient as the pregnancy progresses.
It is very important for pregnant people to avoid fetotoxic substances, like alcohol, nicotine, and certain medications, as the placenta is permeable to these, and they can adversely impact fetal development, potentially causing serious conditions such as fetal alcohol spectrum disorders (FASD).
A fully developed placenta has a large network of blood vessels from both the mother and fetus that exchange substances without direct blood mixing. This developmental process is usually complete by weeks 14 to 16 of pregnancy.