The uterus is an internal organ made of layered muscles. During labor and birth, the uterus is responsible for dilating and effacing the cervix (the opening of the uterus, see below) and pushing baby down and out by contracting continuously and progressively. Contractions in the uterus are similar to the way your muscles contract, or “flex,” when you curl up your bicep. Throughout pregnancy, the uterus provides a safe space for baby to grow and a place for the placenta (which nourishes baby) to attach. Outside of pregnancy, the uterine walls grow a thick lining that is shed every month during menstruation, from the time of puberty to menopause.
The cervix is the tip or opening of the uterus and is made of different tissue than the rest of the uterus. It is internal and cannot be seen unless using a speculum. The cervix is what dilates (opens) and effaces (thins) to allow passage of the baby in birth. The cervix dilates and effaces as the uterus contracts and pulls the cervix upward. During pregnancy, the cervix is thick and closed.
The vagina, contrary to what most people believe, is an internal structure. During birth, it is the “hallway” or “canal” that baby passes through after the cervix (tip/opening of the uterus) and just before being born. The vagina is stretchy as it’s made of many folds (called rugae) that unfold as baby passes through. The vagina is different from the external/visible genitals, which is called the vulva and includes the mons pubis, labia, clitoris, urethral opening, and perineum.