Sex on first day of ovulation could i get pregnant? withdrawal.

Unlikely: It's about a 20% chance that you would get pregnant on this day of your cycle. If you don't want to be pregnant consider using two forms of birth control. At 16 that would be a good idea. Condoms are always a great idea as they also help to prevent STDs

High probability: to get pregnant around ovulation and withdrawal is not an effective form of birth control.