Maybe a connection to birth control drugs too.
I am only thinking of things western women would have access to the women in the poorer countries may not have access to.
Women who are in high paying careers have a high rate of breast cancer — they have a low rate of having kids.
Women who are lesbians and nuns have the highest rates of breast cancer — again, low fertility.
The career women generally are using artificial birth control while the lesbians and nuns are not using any form of birth control.
So the variable that seems to connect them all? Low fertility.