You Grow an Organ: The Miraculous Placenta
What happens: After conception, the fertilized egg meanders to the end of your fallopian tube and enters your uterus, searching for the uterine wall. If it successfully implants itself there, the placenta will begin to form at that very spot about a week later. By the end of the first trimester, the placenta is fully grown and weighs about 11/4 pounds.
Why it happens: The placenta does many things, but its most important task is to act as the bridge between baby and mom. It's responsible for filtering and exchanging nutrients and gases and -- in its job as an endocrine organ -- for making and secreting many hormones. The placenta also serves an immunity role, helping to pass along mom's immune cells before the fetus can develop them by itself.
