The possibility of pregnancy exists when you have sex once without contraception. Contraception means preventing the male sperm from joining the female’s egg. Fertilization of the female’s mature egg begins when it comes into contact with the sperm, which then signals the beginning of pregnancy, i.e., the onset of gestation. By having sex without contraception, there is a high chance that the sperm will come in contact with the egg and fertilization will occur. Any time you have sex without contraception, there is a possibility of pregnancy. If you are not ready to get pregnant, you should take precautions before having sex, such as wearing a condom or taking birth control pills as prescribed by your doctor.