Final answer:
The fetus is protected from most bacterial invasions by the placental barrier, which is a part of the placenta facilitated by the chorionic villi.
Step-by-step explanation:
When lecturing to a class of prenatal women, the nurse explains that the fetus is protected from most bacterial invasions by the placental barrier. The placenta is a lifeline that develops between the embryo and mother, allowing for the transfer of substances between them while also acting as a protective barrier. The chorionic villi, part of the placenta, increase the surface area for the two-way exchange of substances between maternal and fetal blood but do not allow for the exchange of blood cells, providing a mechanism for protection against many pathogens.
Although the yolk sac supplies some nutrients to the embryo in early development, its main role is to provide blood cells and germ cells once the placenta takes over at about the fourth week. The amnion is a sac filled with fluid that cushions and protects the fetus but does not offer protection against bacterial invasion. The chorion develops into the fetal portion of the placenta, with the chorionic villi playing a significant role in nutrient and waste exchange and, importantly, in providing a barrier against infection.