It is possible to get pregnant three days before your period, but not very likely. Most women have already ovulated and are not able to conceive immediately before their period.
Pregnancy occurs when an egg is fertilized by a sperm cell. Women have a three or four day window for conception each month, around the time they ovulate.
Your Fertile Days
Women with a regular, 28-day cycle usually ovulate between day 12 and 16 of their menstrual cycle. The first day of your period counts as the first day of a new cycle.
It is possible to ovulate later than usual. This is more likely if you have shorter or irregular periods.
How to Tell if You Are Pregnant
If you had unprotected sex immediately before your period, there is a slim chance you could be pregnant. Take a home pregnancy test if you are concerned.
It is not a good idea to have sex before your period if you want to avoid pregnancy. You are less likely to get pregnant right before your period, but it is still possible. Talk to your doctor to find the right method of contraception for you.