It’s time for a science lesson but we’ll keep it short and simple.
Pregnancy happens when an egg is fertilised by sperm and the zygote (that’s what you call it when they have joined together) implants itself into the lining of the female uterus.
Semen is the white, sticky fluid that holds hundreds of millions of sperm that comes out of a guy’s penis when he cums. When a guy and a girl have unprotected sex (or your protection fails), semen finds its way into the vagina – ever heard the term ‘strong swimmers’?
The sperm swim through into the fallopian tubes. If the girl has recently ovulated (which put simply means releasing an egg from the ovaries), then the sperm can join with the egg. This is called fertilisation – and BAM there you have it … pregnancy!