The Withdrawal Method, sometimes referred to as the Pull Out Method, requires strict adherence in order to work perfectly. Also, because pre-ejaculate can contain sperm, even if you use it perfectly, it’s still possible to conceive.
With perfect use, where the penis is removed and ejaculation happens away from the vagina and vulva completely, around 1 in 25 couples will become pregnant within the course of a year (Hatcher RA, Trussell J, Nelson AL, Cates W, Stewart FH, Lowal D. Contraceptive technology. 19th ed. New York (NY): Ardent Media; 2007). With typical use, which is more common, around 1 in 5 couples using the withdrawal method will become pregnant (Kost K, Singh S, Vaughan B, Trusell J, Bankole A. Estimates of contraceptive failure from the 2002 National Survey of Family Growth. Contraception 2008; 77: 10-21.)
The Withdrawal Method works best when combined with another method of contraception. This can mean using Fertility Awareness to check when your ovulation times are and only relying on Withdrawal during times when you know you aren’t fertile. Or using it in conjunction with a Diaphragm or the Mini Pill.