How about a pregnancy progression system? Like, the girl going through the first, second, and third trimester gradually. Maybe a couple of weeks, a few months (whatever that's like in the game). After "cumming inside" there's an indication that a pregnancy might be possible. If the impregnation is successful, the girl may tell the player after a few days. A few opportunities for pregnant sex might be a thing.
On the more complicated side:
Maybe build a nursery at home base? Limit the amount of babies that can be had?
During the pregnancy itself, the girl's behaviour may change. For example, they might gather less resources than normal or not want to help at all due to sickness, or the girls will only eat a specific type of food.
At the last stretch of the pregnancy the girl will say that she's feeling a lot of pain. At this point you can trigger the labor sequence (that is if you want childbirth in the game) but only if two conditions are met: 1) Build the nursery; 2) The thrid trimester is at it's end; once both conditions are met, the player can trigger the labor process. It is possible to prolong the pregnancy but not without a few negative side effects. For one, the player will be denied sex. Secondly, the girls would be more irritable, like the cat will take 10 points off the player's health until her pregnancy is over.
The labor scene could have it's own mini-game, like the sex mini-game except the player is helping the girls to control their breathing and easing their pain.
