1. The game has a time system which goes all the way up to a year
2. Each girl has a specific fertility period, in which they can only get pregnant during a certain window of time.
3. I've simplified it to be only a set number of days within each month. So for one girl, it might be on days 11-14, and another it might be on days 20-21.
4. Each girl will announce to the player that they're ovulating and that "its dangerous" etc.
5. Each girl has a fertility level, and the player has a virility score. Once you deposit cum inside their womb, a check is run at the end of each day comparing both the amount of cum, your and her virility, against a base pass or fail depending on the girl. If the number is greater than the pass/fail stat, she will announce the pregnancy to you at the end of the day (hopefully, unless I borked the code and it skips the first time, in which you'll be notified about the birth once the gestation period has finished. )