Explain why the menstrual cycle stops during pregnancy.
The menstrual cycle stops during pregnancy because the fertilised egg develops into an embryo and then a foetus, which must be nourished inside the uterus.
When an egg is not fertilised, the inner lining of the uterus—which is thick and rich with blood vessels—sheds, causing menstruation. However, during pregnancy, this thick lining is needed to receive and nourish the developing baby.
Because the uterine lining is actively supporting the pregnancy, it does not shed. Consequently, the regular cycle of ovulation and menstruation pauses until after childbirth.
Explanation
The textbook explains that menstruation is the shedding of the inner lining of the uterus, which occurs when an egg is NOT fertilised. If the egg is fertilised, it implants in this thick, blood-rich lining, which is then required to protect and nourish the developing embryo and foetus throughout the pregnancy. Since the lining is needed, it does not shed, thereby stopping the menstrual cycle until the baby is born.