At some point years of education crimps into childbearing years - I know two women who could easily have obtained their degree even after having a child, but quickly got to a “fuck this shit I’m a mom” and left the program never to return.
I don’t even know if things like FMLA apply to college classes.
This was my theory initially but it doesn’t pan out due to the decline being observed among all income and education levels; you would also expect to observe a steeper decline among women with advanced degrees, but women with master’s degrees and doctorates actually have a slightly higher fertility rate than those with just a bachelor’s degree. Taking even 5 years at college still gives a woman around 15 to 20 years to find a partner and have children.