The Withdrawal Method
Just as the rhythm method isn't a 100 percent guarantee against conception, the withdrawal method, in which a man removes his penis from a woman's vagina before ejaculation, doesn't guarantee that pregnancy won't occur. In fact, it just might lead to a deposit in the baby-making box.
According to Planned Parenthood, even if the method is followed completely accurately every time and the man manages to interrupt coitus before emitting any ejaculate, 4 out of every 100 women will still become pregnant. And, if the method isn't employed correctly each and every time sexual intercourse takes place, the odds rise: 27 out of every 100 women will conceive.
This is because of something known as pre-ejaculate -- an uncontrollable fluid that can appear at the tip of man's penis during sexual stimulation. Because it's believed this fluid can pick up sperm left in a man's urethra from a previous ejaculation, if the fluid appears during intercourse, the sperm it contains can still find their way to an egg, even if the man doesn't have an orgasm during penetration.