The inner lining of the uterus that thickens each cycle and sheds as menstrual bleeding.
Why it matters for PCOS
The endometrium is the lining inside the uterus. Each cycle it thickens under the influence of estrogen to prepare for a possible pregnancy, and if none happens, progesterone falls and the lining sheds as a menstrual period. In PCOS, infrequent ovulation means less progesterone is produced, so periods can be absent for long stretches and the lining stays exposed to estrogen without being shed regularly.
Because of this, a doctor may suggest ways to bring on regular bleeds, such as a progestogen or a combined pill. If you have no period for more than three months, very heavy bleeding, or any bleeding after menopause, an ultrasound or further assessment of the lining may be needed, and the result is interpreted in the context of your overall picture.
This glossary is education, not diagnosis. For your own situation, ask a doctor or pharmacist.