It's all about guessing and estimates. " woman gets her period usually once every 21-40 days. In between her periods, the woman ovulates (she releases an egg). If the egg makes a love-connection with any sperm, it gets fertilized and she might be pregnant. However, if the egg doesn't hook up with any sperm, the egg leaves the woman's body and she gets her period like usual--no pregnancy. Sperm leave a guy's body in his and pre-. In order to fertilize an egg, the guy must ejaculate the sperm inside the woman's vagina.
So, women are most likely to get pregnant around the time they release an egg. Ovulation happens around a week after your period until a week before your next period. But don't count on it! Your period might not be that regular, and ovulation can happen at any time, even during your period. Plus, sperm can live in a woman's body for days waiting for the egg!"
Pregnancy
If this is about paternity - and I assume it is - you have to wait until the baby is born and DNA can be conducted.
"Pull out," of course, is not effective.