Explain why the menstrual cycle stops during pregnancy.

The menstrual cycle stops during pregnancy because the fertilised egg implants in the thickened lining of the uterus and starts developing into an embryo. The uterine lining, which is rich in blood vessels, is maintained to support the developing embryo, and later the placenta helps provide nourishment to the embryo/foetus.

Under normal conditions, if fertilisation does not occur, the thickened uterine lining is no longer maintained. It breaks down and is shed through the vagina along with blood and mucus as menstrual flow. However, during pregnancy, hormonal changes, mainly involving progesterone and estrogen, maintain the uterine lining and prevent its shedding. These hormones also prevent further ovulation. Therefore, menstruation stops during pregnancy and resumes only after childbirth, though the timing may vary.