Eli5: How come nulliparity vs having 1-2 pregnancies(most women have) create a difference in risk of reproductive cancers?

Question

Women spend most of their lives without pregnancy, and when they’re pregnant, it’s usually 1-2 years of their whole life.

Shouldn’t it be not that important when it comes to elevated risk of reproductive cancers (breast, uterine, ovarian etc) if the woman is nullipar?

What’s the explanation?

In: Biology

Answer 1

It’s a relative risk difference. We can’t say for sure how many ovulation cycles are correlated with getting reproductive cancer but we know that things that decrease the # of ovulation (late menarche, early menopause, pregnancy, etc) are protective. So it’s just speaking in relative terms.