Ovulation is when a mature egg is released from the ovary. The egg then moves down the fallopian tube where it can be fertilised. If sperm are in the fallopian tube when the egg is released, there is a good chance that the egg will be fertilised, creating an embryo, which can grow into a baby.
Pregnancy is technically only possible if you have sex during the five days before ovulation or on the day of ovulation. But the most fertile days are the three days leading up to and including ovulation. Having sex during this time gives you the best chance of getting pregnant.
By 12-24 hours after ovulation, a woman is no longer able to get pregnant during that menstrual cycle because the egg is no longer in the fallopian tube.
There’s almost no chance of getting pregnant if you have sex before or after the fertile window (but if you’re not trying to get pregnant, don’t rely on this – contraception is your best option!).