Question

Describe the menstrual cycle. Why does it occur?

Answer 1

The menstrual cycle is a recurring series of changes in the ovaries and uterus that prepares the body for a possible pregnancy. It averages about 28 days, although the length can vary. Day 1 is the first day of menstrual bleeding.

1. Menstrual phase (about days 1–3/5): If pregnancy has not occurred, the levels of ovarian hormones fall. The thickened inner lining of the uterus (endometrium) breaks down and is shed with blood through the vagina.
2. Follicular phase (roughly days 1–13): A follicle in an ovary grows and an egg develops within it. The follicle produces oestrogen, which helps rebuild and thicken the endometrium.
3. Ovulatory phase (around day 14 in a 28-day cycle): The mature follicle releases an egg from the ovary. This release is called ovulation. The egg enters the oviduct (fallopian tube).
4. Luteal phase (roughly days 15–28): The emptied follicle becomes the corpus luteum, which produces mainly progesterone, as well as oestrogen. These hormones maintain the uterine lining in readiness for implantation. If fertilisation and pregnancy do not occur, the corpus luteum regresses, hormone levels fall, and the lining is shed—starting the next menstrual period.

The cycle prepares an egg for possible fertilisation and prepares the uterus to receive and support an embryo. If pregnancy does not begin, the prepared uterine lining is no longer needed and is shed. During pregnancy, the lining is maintained, so menstruation does not occur.