Since nearly everyone here seems to be completely incapable of typing "birth control rates" into google, here are the statistics the fda uses on the withdrawal method, compared to some other common ones. Now, the source has the lowest rate of pregnancy for each method, and the typical rate of pregnancy. I'm going to post the typical rate because I know that half of the posts here are about slipups involving withdrawal.
No method of contraceptives = 85%
withdrawal = 19%
birth control pills = 5%
condoms (without spermicide) = 14%
now, the percentage rate is not how many times you can have sex and not get pregnant. It's what percentage of women get pregnant using said method within one year. So, of 100 women using withdrawal, 19 get pregnant within a year and so forth. That's almost one in every five. And that was the lowest statistic that came up.
Some sources say it may be as high as 30%, which is one out of every three. Effective? No. Risky? Yes.