You can’t get pregnant after having sex right after your period, but you can’t completely rule out that you won’t get pregnant, it’s just that the chances are lower.
The chances of getting pregnant will only be greater if you have intercourse during the ovulation period, which is around the 14th day before the next period, and usually the first 5 days and the last 4 days of this day are called the ovulation period. And menstruation just after the normal is a safe period, so menstruation just after sex is not pregnant, but can not be completely ruled out will not be pregnant, because the ovulation period will be affected by a variety of factors and some advance or push back.
If you don’t want to get pregnant, it’s best not to use the safe period method of contraception, it’s best to do a good job of reasonable and healthy contraceptive measures in every coitus, because the safe period is not completely absolutely safe.