A woman is most fertile and has the highest chance of getting pregnant during the time that you Ovulate.
When you ovulate an egg is released from your ovaries which happens between 12 to 14 days before your next period starts.
A woman has less chance of getting pregnant if they have sex right after their period but it can and has happened but you're most fertile and most likely to get pregnant 12 to 14 days before your next period.
So if you want to increase your chances of getting pregnant you should wait 12 days after your period before trying to get pregnant.