Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Placental mammals get their name from the placenta. This is a spongy structure that develops during pregnancy only in placental mammals. You can see where a human placenta forms in Figure below . The placenta sustains the fetus while it grows inside the mother's uterus. It consists of membranes and blood vessels from both mother and fetus. It allows substances to pass between the mother's blood and that of the fetus. The fetus gets oxygen and nutrients from the mother. It passes carbon dioxide and other wastes to the mother. Answer: placenta
What sustains the fetus while it grows inside the mother's uterus?