Since the fetus is enclosed in the mother's womb, the lungs are not exposed to air. Nor can the fetus eat normally through the mouth. The fetus must rely on oxygen and nutrients from the mother to survive.
Oxygen and nutrients are received from the mother through the placenta and umbilical cord. Blood from the mother's placenta is carried to the fetus via the umbilical vein. About half of the blood goes directly to the heart. The rest goes to the liver. From here the blood enters the fetus' circulation system. The fetus' heart, not the mother's heart, then pumps the blood through the fetus circulation system.
Another major difference between a fetus and born human, is that the fetus has an opening between the right atrium and left atrium. Most blood entering the fetal heart flows directly from the right atrium to the left atrium. This bypasses the pulmonary circulation system, where a born human's blood would be pumped to the lungs for oxygenation. Blood is pumped directly into the systemic circulation system to deliver oxygen and nutrients fetal organs, tissue and cells.
In addition to relying on the mother for oxygen and nutrients, the fetus must also rely on the mother to dispose of waste. Some of the blood from the fetal circulation system, when passing through the internal iliac arteries, enters the umbilical artery. This blood re-enters the placenta where carbon dioxide and other waste products are removed before returning to the fetus. These waste products then enter the mother's circulation system for disposal.