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

Passage: Within seconds of birth, the umbilical cord is cut. Without this connection to the placenta, the baby can't exchange gases, so carbon dioxide quickly builds up in the baby's blood. This stimulates the baby's brain to trigger breathing, and the newborn takes her first breath. Answer: umbilical cord
What structure connects the fetus to the placenta, allowing the exchange of gases?