The time when someone with a uterus typically has the highest sex drive, namely ovulation, is also the time that they are most able to get pregnant from PIV. Even if you’ve taken all other contraceptive precautions (condoms, both control, etc), there is still always at least a minuscule chance that those contraceptive measures will fail, so if you want to have sex and really make sure they succeed, the optimal time to prevent conception is during your period. Unfortunately, for many with a uterus, this is also the time that sex drive is lowest, and many people with penises are averse to the idea for one reason or another. However, assuming both partners are willing to have PIV while the uterus-owner is on their period, they’ll be the least likely to get pregnant compared to any other phase of the cycle.