The simple answer is that it is possible to get pregnant from having sex on a period.
Your menstrual period and menstruation is falsely believed to be a "safe" period of time in your monthly cycle when you can have unprotected intercourse and not get pregnant. This is a myth, and nothing is further from the truth.
You can still get pregnant on your period if you have unprotected sex. Sperm survive up to five days, and some women can ovulate within 5 days of their menstrual period. And if you make love within 5 days of ovulation then it is possible to get pregnant.
Sperm have the amazing ability to live for up to 5 days inside of the female body, which is why you can get pregnant on your period. This amazing survival time means that you can actually get pregnant -- that the egg gets fertilized -- on a different day than when you had sexual intercourse.
That sounds strange, but it is true!
If you have sexual intercourse while you are on your period, and the sperm stays alive for 4-5 days inside of you, and then you ovulate while the sperm is still there, you can get pregnant on your period.
Some women can not actually get pregnant on their period, because ovulation happens more than 5 days after their period ends. But others, due to the timing of their periods and earlier ovulation, can get pregnant on their period.
Women with shorter menstrual cycles ovulate earlier. For example, those with menstrual cycles of 21 days will ovulate usually on cycle day 7, which is just a few days after your menstrual period ends.
The fact is that you can get pregnant on your period. This is why it is important to continue to use your birth control method all the time, even when having intercourse while having your period.
If you are on the birth control pill or a vaginal hormone ring that is removed during menstruation, you are still covered while on your menstrual period. The hormones do not leave your body in that period of time, as long as you use your method as directed.
And birth control pills cannot protect you from sexually transmitted infections or HIV, so using condoms at all times is a smart idea. Even though you are unlikely to get pregnant on your period, you still have the same risk of contracting a sexually transmitted infection, including HIV.