Exercise 11.5
Explain why the menstrual cycle stops during pregnancy.
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
Because during pregnancy the thick uterine lining is needed by the embryo, so it is not shed — and menstruation is the shedding of that lining.
The lining becomes thick and rich in blood vessels in every cycle in order to receive and nourish a developing zygote.
When the egg is not fertilised, it degenerates within about a day, the lining is no longer needed, and it breaks down and leaves the body with some blood as menstruation.
When fertilisation does happen, the zygote implants into that very lining and draws its nourishment from it. Shedding the lining would mean losing the embryo.
So the cycle of ovulation → thickening of the lining → shedding is interrupted, and menstruation resumes only after the pregnancy is over.