I vote pregnancy.
Either a slider for an estimation for how far along she is, first trimester, second, third... and super huge...
Pregnancy should be free, it's the natural course of events, and technically most women can birth without a hospital. However, as someone else pointed out, there could be ways to make money: pills, condoms, etc... Each having their various chances of failure. The bigger the cost, the less chance of failure. On the other hand there could be fertility things can buy to make you more likely to get pregnant.
Maybe have an optional thing where every baby that you successfully birth gives you a pink or blue token on your profile for babies that you've had.