Manila is not able to get pregnant. That part is not implemented into game.
I think it is more likely to be something like ending than part of active gameplay.
Dunno, from the CG there's versions of her in each outfit in varying stages of pregnancy. I think originally at least the idea was that she could get pregnant and it would progress over the course of the game. Hard to see how it would work though, without either a) a magic pill that shortens the gestation period to a matter of days or b) the games narrative structure changes completely, so you get several cases over the course of a couple of years in her career, with big skips between each one allowing for the pregnancy to advance.
I actually think option a would be significantly more likely, though it is ridiculous of course. The country is known to be having some major fertility problems - if I recall correctly Manila is rare in actually being fertile, with the implication that fertile woman will need to have several times the number of children as those in our world in order to keep the population stable. A pill that shortens the gestation period would help a lot.
Furthermore any society like this would doubtless have a lot of infertile couples looking to adopt. So once Manila's given birth the usual problem for games with pregnancy systems - namely what does the mother do with the baby - is easily solved, without having to introduce a nursery system or have some ever increasing accumulation of cots in the corner of Manila's living room.
In fact you'd expect a black market to emerge of couples willing to pay to adopt, and once you take that into account another piece falls into place - the villain has a particularly keen interest in her, considering her (again if i recall correctly) perfect for his plans, especially after finding out more about her physiology. Perhaps rather than simple street walking like J recommends, the plan is for a more...consequential... form of prostitution.