Train Constitution (using time)

You decide to dedicate your time to bettering your body's resilience, paying the fee daily until your body strengthens itself. The first day you pay the fee, and are given a towel to cover anything you might want covered, you being allowed entry. You start in a steam room, your body soaking in the restorative steam. After a while there, you then shift to an ice room, realizing that the towel is there for more than mere decency as it quickly freezes to your body, you suppose enhancing the experience, although right now you're not overly appreciative of the ingenuity of the creators of this establishment.

You continue doing this day to day, eventually culminating in a body that definitively feels like it can take more of a beating. The passage of time also has definitively resulted in a mind which is aware of its body's pregnancy.