How does a fetus obtain oxygen?

Fetuses need oxygen because (like us) they are made up of cells that use aerobic metabolism. Aerobic metabolism is a system used by cells to make cellular energy out of glucose (sugar) and Oxygen. Fetuses, which start out as one unique cell, need oxygen because they have the same machinery.

If for some reason they didn't get enough oxygen they (like us) would start producing acid and 1/16th of the energy they were producing before and the cells would die.