Final answer:
The endometrium helps form the placenta around the fetus, working alongside embryonic tissues. This development creates the placenta, which facilitates nutrient and gas exchange through blood vessels connected by the umbilical cord.
Step-by-step explanation:
The structure in the mother's body that helps form the placenta around the fetus is the endometrium. The endometrium, along with the embryo's tissues, contributes to the growth and development of the placenta.
The placenta is an essential temporary organ that begins to form from the trophoblast layer of cells shortly after implantation. It consists of a spongy mass of blood vessels which includes both maternal and embryonic components, facilitating the vital function of nutrient and gas exchange between mother and fetus. As the pregnancy progresses, the placenta becomes more established, with maternal blood vessels closely intertwined with those of the growing fetus, the separation of which is maintained by a thin barrier allowing exchange without direct blood mixing.
The mature placenta is connected to the conceptus via the umbilical cord, which carries deoxygenated blood and wastes back to the placenta through two umbilical arteries, and delivers oxygenated blood and nutrients from the mother through a single umbilical vein.