Originally Posted by Magnetic
No exact figures are available, but the odds are fairly low. What is your risk tolerance? Remember, even a 5% chance gives 1 in 20 odds of getting her pregnant. So, by the time you have sex with her 20 times she will be pregnant. Decide what you think the odds are (probably 1 to 5%) and decide if it is worth the risk. Decide if that risk is worth not wearing a condom.
Usually, statistics on success or failure rates of particular methods of birth control are given over a period of a year, not in numbers of uses. So when the FDA says the withdrawal method has a 'typical use' failure rate of 19%, they mean that if you use that method for a year, you will have a 1 in 5 chance of becoming a father.
I, personally, wouldn't risk it with those odds...