Based on your menstrual cycle, when's the best time to have sex so you have the lowest chance of pregnancy? – Megan*
The time a girl is most likely to get pregnant is the time about halfway between periods, when ovulation occurs. But trying to time things to a "safe" time can lead to an unintended pregnancy because it can be very difficult to predict exactly when the egg will emerge from the ovary ready to be fertilized. And girls can be fertile for several days around the time of ovulation.
Couples having sex should use a more reliable method to prevent pregnancy, like birth control pills or the patch — as well as condoms to help prevent sexually transmitted diseases (STDs).