Pregnancy can only result if a sperm cell meets an egg cell. For this to happen, a guy must leave his sperm within the vagina or near the vaginal entrance.
This is the case if you have intercourse without a condom. If the guy ejaculates he leaves behind three to five million sperm cells. Most of these leave the vagina as soon as the girl gets up after sex: they slip out of the vagina.
The fastest enter the womb through the cervix. They move upward to the fallopian tubes, where they hope to meet an egg cell.
If ovulation has not yet taken place, they lay waiting for the moment an egg will be released from the ovaries.
If this doesn’t happen within a few hours or days the sperm dies. But if an egg cell and a sperm cell meet: bingo!