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The endometrium is one of the linings of the uterus. It is composed of glands and stroma, which both respond to estrogen and progesterone produced cyclically in the ovary. The endometrium has two zones, the superficial and the deep.

The superficial zone (aka stratum functionalis), which makes up two-thirds of the endometrium, responds to both estrogen and progesterone, and sheds during the menstrual period. The deep zone (aka stratum basalis) is not responsibe to hormones, is not shed, and regenerates after menses.

The endometrium follows the 28-day menstrual cycle, and has three phases, during which ovulation occurs on day 14. The phases are:

  1. Menstrual phase - days 1-4
  2. Proliferative phase - days 5-14
  3. Secretory phase - days 15-28

The endometrium also changes during pregnancy and after menopause.