Final answer:
The placenta is the structure that exchanges oxygen and nutrients between the blood of an unborn child and their mother.
Step-by-step explanation:
The structure that exchanges oxygen and nutrients between the blood of an unborn child and their mother is the placenta. The placenta is a spongy organ that develops jointly from the embryo and uterine wall structures and allows for the exchange of substances between the fetal and maternal blood without mixing them. It passes oxygen, nutrients, and other useful substances from the mother to the fetus, while also removing carbon dioxide and other wastes from the fetus.