Thanks to its ability to change size and shape in response to hormones, the cervix is a bit of an acrobat, says Ronald D. Blatt, M.D., chief surgeon and medical director of the Manhattan Center for Vaginal Surgery. It has two main functions: When you’re not pregnant, it protects you by keeping germs and other stuff (pool water, tampons, sex toys) from going further up inside you. If and when you do decide you want to make a baby, it helps the sperm get to the egg and, once you’re pregnant, helps keep the baby inside the uterus. Then when you go into labor, it opens up to let the baby out.

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