I was wondering why we were getting this question a lot lately. Then last week, I saw the movie 'Knocked Up.' Lo and behold, one of the characters in the movie claims (and exclaims) that a girl can't get pregnant if she's on top. 'Cause of gravity and all that.
WRONG! So totally not true!!!
Yes, women can get pregnant if they are on top and the man ejaculates into her. Granted, yes, gravity works against the bulk of the semen, and a lot of it can dribble back out. But there is usually enough residue in there to allow some sperm to make their way into the reproductive tract. On a very microscopic level, there are special mucus proteins in the cervix that help guide the sperm up into the body as well as tiny "fingers" called cilia on some of the epithelial cells in the reproductive tract that work to sweep sperm towards the upper reproductive tract. Remember folks, nature wants you to reproduce. That doesn't mean it'll happen at the most opportune times. But nature has been around far longer than you or me, and a simple ploy like waiting for gravity to cause sperm to leak out of the woman isn't going to outsmart or suffice against millions of years of evolution.
Instead, if you really want to prevent pregnancy, the man needs to wear a condom, and the woman can use a second method of birth control like hormonal birth control pills, a diaphragm or cervical cap.
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