How does the endometrium change during the menstrual cycle?

The endometrium thickens and sheds during the menstrual cycle in response to changing hormone levels.

The menstrual cycle, which lasts approximately 28 days, is a series of changes in the female reproductive system that prepares the body for potential pregnancy. One of the key components of this cycle is the endometrium, the inner lining of the uterus. The endometrium undergoes significant changes throughout the menstrual cycle, driven by fluctuations in the levels of the hormones oestrogen and progesterone.

At the start of the menstrual cycle, the endometrium is relatively thin. This is because it has just been shed during menstruation, a process in which the thickened endometrium from the previous cycle is expelled from the body. This shedding of the endometrium is what causes the bleeding associated with menstruation.

Following menstruation, the endometrium begins to thicken again. This is the proliferative phase of the menstrual cycle, and it is driven by increasing levels of oestrogen. The oestrogen stimulates the cells of the endometrium to multiply and the blood vessels to expand, causing the endometrium to thicken. This thickening prepares the uterus for the potential implantation of a fertilised egg.

If fertilisation does not occur, the levels of oestrogen and progesterone decrease. This triggers the start of the secretory phase, during which the endometrium continues to thicken and the glands within it begin to secrete substances that would nourish a fertilised egg. However, if no fertilised egg implants in the endometrium, the hormone levels continue to fall.

This drop in hormone levels at the end of the menstrual cycle triggers the shedding of the endometrium, marking the start of menstruation and the beginning of a new menstrual cycle. This cyclical thickening and shedding of the endometrium is a key aspect of the menstrual cycle and is crucial for female fertility.