The inner lining of the uterus that thickens each cycle and sheds as menstrual bleeding.
What the endometrium is
The endometrium is the inner lining of the uterus, the surface that covers the space where a pregnancy could develop. This lining does not stay the same all the time. It changes through the monthly menstrual cycle, sometimes thickening and sometimes shedding. These changes are controlled by two main hormones, estrogen and progesterone, which rise and fall at different points in the cycle. Understanding the endometrium helps explain why periods happen and why their pattern can change.
How it changes each cycle
Early in the cycle, the endometrium thickens under the influence of estrogen to prepare for a possible pregnancy. After ovulation, progesterone helps stabilise the lining. If no pregnancy occurs, progesterone falls and the endometrium sheds as a menstrual period. This is why a regular period usually signals a cycle that involved ovulation. To understand the role of these hormones further, see the hormones page.
Why it matters for PCOS
In PCOS, infrequent ovulation means less progesterone is produced, so periods can be absent for long stretches. When this happens, the endometrium stays exposed to estrogen without being shed regularly, and the lining can keep thickening. This is why doctors pay attention to the menstrual pattern in women with PCOS, because very infrequent periods over a long time can affect the health of the uterine lining. Understanding this helps explain why tracking the menstrual cycle matters, and is not simply a question of monthly comfort. The link between irregular ovulation and periods is explained further on the what is PCOS page.
When to talk to a doctor
Because of these concerns, a doctor may suggest ways to bring on regular bleeds. These options should be discussed personally with a doctor rather than started on your own, since what suits one person may not suit another. If you have no period for more than three months, very heavy bleeding, or any bleeding after menopause, a doctor may suggest an ultrasound or further assessment of the lining, and the result is always interpreted in the context of your overall health rather than as a single finding on its own.
This glossary is education, not diagnosis. For your own situation, ask a doctor or pharmacist.