I took a biology class this summer that was about pregnancy, and it covered everything from having sex to giving birth. It was really interesting, but one of the controversial subjects covered was the exact moment when the fetus becomes a living being.
So when does pregnancy begin? Many people have lots of different opinions, but Planned Parenthood puts it in the easiest terms for people (like myself) who aren’t biology majors.
Their page on How Pregnancy Happens even has a cool flash animation about it.