Which of the following are the primary functions of the placenta? Select all that apply

The amnion is a thin, transparent membrane that forms the innermost layer of the fetal membranes, creating a protective sac filled with amniotic fluid. By 12 weeks gestation, the amnion has expanded to enclose the fetus completely, providing a cushioning environment, temperature regulation, and protection against infection and mechanical injury. Amniotic fluid volume normally ranges from 50 ml at 12 weeks to approximately 800 ml at term. The amnion does not produce hormones or directly facilitate nutrient exchange, which is the role of the placenta.

Rationale for correct answers

B. The amnion forms a protective sac that contains amniotic fluid, which cushions the fetus and permits movement. At 12 weeks, this sac is fully formed, surrounding the embryo/fetus with fluid essential for normal development and mechanical protection.

Rationale for incorrect answers

A. The amnion does not produce hormones; hormone production to maintain pregnancy is primarily by the trophoblast and placenta, including human chorionic gonadotropin and progesterone.

C. Nutrient exchange occurs at the placenta via chorionic villi, not the amnion, which acts as a barrier and container for amniotic fluid but does not participate in exchange functions.

D. The umbilical cord develops from the connecting stalk and allantois, not from the amnion. The amnion lines the amniotic cavity but does not differentiate into the umbilical cord.

Take home points

• The amnion forms a protective sac filled with amniotic fluid by 12 weeks gestation.
• Amniotic fluid cushions and protects the fetus, volume increases from 50 ml at 12 weeks to 800 ml at term.
• Hormone production for pregnancy maintenance occurs in the placenta, not the amnion.
• Nutrient and gas exchange occurs through the placenta, not the amnion.

The placenta is a vital organ for fetal development, facilitating maternal-fetal exchange. It performs gas exchange, nutrient delivery, and waste removal between maternal and fetal blood without direct blood mixing. The placenta also transfers immunoglobulins, mainly immunoglobulin G (IgG), providing passive immunity to the fetus. It produces hormones essential for pregnancy maintenance but does not produce fetal hemoglobin or regulate fetal blood pressure directly. Normal placental weight ranges from 400 to 600 grams at term.

Rationale for correct answers

A. The placenta facilitates the transfer of oxygen from maternal blood to fetal blood via diffusion through the placental membrane, critical for fetal oxygenation.

B. It acts as an interface for the excretion of fetal waste products such as carbon dioxide and urea into maternal circulation for elimination.

C. The placenta transfers maternal immunoglobulins, especially IgG, across the syncytiotrophoblast to the fetal circulation, providing passive immunity.

Rationale for incorrect answers

D. Fetal hemoglobin (HbF) is synthesized by the fetal liver and bone marrow, not by the placenta.

E. Regulation of fetal blood pressure is controlled by fetal cardiovascular and renal systems, not directly by the placenta.

Take home points

• The placenta transfers oxygen and nutrients and removes fetal wastes.
• It mediates passive immunity by transferring maternal antibodies to the fetus.
• Placenta does not synthesize fetal hemoglobin or regulate fetal blood pressure.
• Placental function is essential for fetal survival and growth.