Mittelschmerz (German: "middle pain") is a medical term for "ovulation pain". Some women have mittelschmerz regularly and can time their ovulation by it but many never experience it.

It is characterised by lower abdominal and pelvic pain that occurs roughly midcycle in a woman's menstrual cycle. It is thought that this is caused by irritation of the peritoneum by bleeding associated with rupture of a follicular cyst during ovulation. (The ovaries have no openings and at ovulation the egg breaks through the ovary's wall.) The pain can appear suddenly but usually subsides within hours and may be accompanied by a small amount of vaginal bleeding. In some women, the mittelschmerz is localized enough so that they can tell which of their two ovaries provided the egg in a given month.

In theory, going on the pill (oral contraceptive pill) should remove this pain altogether as it prevents ovulation.

The pain of mittelschmerz is sometimes mistaken for appendicitis and is one of the differential diagnoses for appendicitis in women of child-bearing age.