The thing is though, women are most horny around ovulation, and therefore are more likely to have sex near ovulation
Also not all women have a completely regular cycle, and does OP even know where she is (thinks she is) on the cycle?
There's also the fact that if there's twins in her family she may release multiple eggs, giving more chance of one of them implanting
I'm not saying that "a droplet of pre-cum is going to get someone pregnant every time" but penis-in-vagina sex without birth control has a possibility of resulting in pregnancy
So unless OP *wants* to have a kid, he needs to get Plan B right the fuck now