During your menstrual cycle, the female sex hormone estrogen causes your uterine lining to thicken every month to create a nourishing environment for a fertilized egg. Soon afterward, a follicle — a tiny sac in your ovary that contains a single egg — ruptures and releases its egg (ovulation).

If the egg becomes fertilized on its way to your uterus by contact with a sperm, the egg implants in the lining of the uterus. However, most often the unfertilized egg passes through your uterus and out of your body. Shortly thereafter, your uterus releases this lining and your menstrual flow begins.

Mittelschmerz occurs during ovulation, when the follicle ruptures and releases its egg. It's estimated that 1 in 5 women experience ovulation discomfort. Some have mittelschmerz every month, while others have it only occasionally.

The exact cause of mittelschmerz is unknown, but possible reasons for the pain include these:

  • Just before an egg is released with ovulation, follicle growth stretches the surface of your ovary, causing pain.
  • Blood or fluid released from the ruptured follicle irritates the lining of your abdomen (peritoneum), leading to pain.

Pain at any other point in your menstrual cycle isn't mittelschmerz. It may be normal menstrual cramping (dysmenorrhea) if it occurs during your period, or it may be from other abdominal or pelvic problems. If your pain is severe during the time of ovulation or at any other time during your cycle, see your doctor.

Jun. 11, 2011