I think pregnancy might have broken in one of the recent updates. I read on Milkynail's official site that it should only take 15 days, but I've had multiple pregnancies last longer than that, and ended up aborting all of them.
But also, from what I've seen, all the people who've had this problem have gotten pregnant by the step-dad, so maybe that has something to do with it?
edit: https://milkynail.site is the place I got that info
edit2: after avoiding sex scenes that would cause pregnancy (only doing anal/lesbian scenes) my pregnancy ended on day 23, so maybe that's the answer. Doing scenes that would get you pregnant resets the timer?