Oxygen is conveyed to the developing fetus via the umbilical cord, which stretches from its abdomen to the placenta, a temporary organ with a large surface area in contact with the mother's uterine lining. According to the National Children’s Study, the placenta also filters potentially harmful substances out of the mother's blood and conveys essential nutrients to the fetus in the womb.
Oxygen is present in the air, and it is taken into the mother's lungs with every breath. Once the oxygen is absorbed into her bloodstream, the mother's body circulates the oxygen-rich blood to the uterus, where it passes through a membrane into the placenta, which is filled with blood vessels and receives a steady flow of blood from the baby, relays Wikipedia. Once the oxygen crosses the placental barrier, the fetus' circulatory system transports it to where it is needed in its body.
The counterpoint of this cycle, according to Wikipedia, is that waste gas, such as carbon dioxide, is taken up by the fetus' blood and returned to the placenta, along with other waste, and then passed back to the mother's bloodstream. The exhausted blood is carried to the mother's lungs to discharge the carbon dioxide and take up fresh oxygen, completing the cycle.