How does a developing fetus get oxygen in the womb?

The placenta acts as a lifeline between a mother and her developing fetus, performing functions vital for fetal development. Its primary role is to provide a unique environment where nutrients, gases, and wastes are exchanged without the direct mixing of maternal and fetal blood. The placenta attaches to the uterine wall and contains numerous villi — tiny, blood vessel-filled projections which increase the surface area for effective substance exchange.

Through these villi, nutrients such as glucose and amino acids from the mother's blood are transferred to the fetal bloodstream. Additionally, the placenta secretes important hormones like human chorionic gonadotropin (hCG) and progesterone, which are crucial for maintaining pregnancy and supporting fetal growth. It's the efficiency and sophistication of placental function that allows a fetus to grow and develop in a protective and nourishing womb environment.

Gas exchange during pregnancy is a finely-tuned process that ensures the fetus receives the oxygen it needs to grow while removing carbon dioxide, a metabolic waste product. This exchange takes place at the placental barrier, where the mother's oxygen-rich blood comes into close contact with the fetal blood in the umbilical cord vessels. The process is facilitated by the difference in partial pressures of oxygen and carbon dioxide on each side of the barrier, driving the gases to move from higher to lower pressure areas.

The semi-permeable nature of the placental membrane allows oxygen to diffuse into the fetal blood, while allowing carbon dioxide to pass back into the maternal bloodstream to be expelled. This system ensures that the fetal blood is adequately oxygenated before it circulates through the developing baby's body, providing the necessary fuel for cellular processes and growth.

The umbilical cord is the physical link that facilitates blood circulation between the fetus and placenta. Within this extraordinary cord, there are two umbilical arteries and one umbilical vein. The oxygenated blood from the mother is carried to the fetus through the umbilical vein. In contrast, the umbilical arteries carry deoxygenated blood and waste products from the fetus back to the placenta.

Unlike typical venous blood, the blood in the umbilical vein is rich in oxygen and nutrients. Once it reaches the fetus, it flows through the fetal liver and enters the systemic circulation to be distributed throughout the fetus's body. After the nutrients are utilized and oxygen is exchanged for carbon dioxide by the fetal tissues, the now deoxygenated blood is returned to the placenta through the umbilical arteries. Remarkably, blood circulation via the umbilical cord is the foundation that supports the growth and survival of the fetus until birth.