Pre-ejaculate is released all through the sex act and it enters your partner's vagina. Any sperm in it can travel up through the cervix and find the egg in the fallopian tube.
The withdrawal method (also called coitus interruptus) refers to the withdrawal of the penis from the vagina before ejaculation. It's an ancient method, costs nothing, and has no side effects. But it is undependable. A pregnancy may happen because of sperm in the pre-cum.
Some studies have shown that more than 40% of men have sperm in their pre-ejaculate fluid. Some men always have sperm in their pre-ejaculate, while others never do. This always or never phenomenon probably explains why some men are successful at birth control by the withdrawal method. But the possibility of sperm in pre-ejaculate is high. The way to avoid pregnancy is to wear a condom from the first moment of genital contact.
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WebMD, "What Is Pre-Ejaculate?"