Human births take a much longer period of time than monster births, about double that of nekomatas and triple that of imps, according to the code, or anywhere from 30-45 days after you enter stage 2 (full belly) which itself takes 15ish days to enter.
Part of the issue might be the variance that human pregnancies have compared to monster ones. Imp pregnancy always takes 28 days according to the code, but human pregnancy can be as short as 52 days or as long as 72! days.
The enhanced length of the pregnancy is supposed to tie into some more in-depth pregnancy events around human pregnancies and separate them from monsters, which grow rapidly in comparison.
I think the birth scene itself might be broken at the moment(I think I was redoing artwork for them and needed to make some new ones for it), but the pregnancy will resolve after a lengthy wait, and the missing scenes will be implemented once I can finish up the art for them.