Pregnancy sac
From Tardis Wiki, the free Doctor Who reference
A pregnancy sac was an organ possessed by Gifftans in which their young was gestated. During childbirth, the pregnancy sac was cut open painlessly as the child was born. (TV: The Tsuranga Conundrum).
Due to its role in pregnancy, it may have been the equivalent to the womb in humans. (TV: The Lost Boy)
