Sex on your period
Your menstrual period is falsely believed to be a "safe" time during your monthly cycle when you can have unprotected sex and not get pregnant. This is actually a myth, and nothing is further from the truth. The simple answer is that it is possible to get pregnant from having sex while you have your period.
How it happens
Sperm has the amazing ability to live for up to 5 days inside of the female body. This amazing survival time means that the egg can get fertilized on a different day than when you had sex. That sounds strange, but it is true! If you have sex while on your period and then ovulate a few days later, you can easily get pregnant.
Some women can ovulate within 5 days of their period and get pregnant from having sex that took place during their menstrual bleeding. Any time you have sex within 5 days before ovulation, it is possible to get pregnant.
Can you get your period once you are already pregnant?
It is possible but highly unlikely to have a regular menstrual period once you are pregnant. When you're pregnant, you may have some spotting or bleeding, but it's rarely a regular menstrual period. A regular menstrual period is moderate to heavy bleeding, lasting 3-5 days. Up to 30% of pregnant women experience some small amount of spotting or vaginal bleeding early in pregnancy (and most go on to have a normal pregnancy).
Some even report intermittent bleeding that seems like a regular period to them, but vaginal bleeding during pregnancy is not the same as menstruation.
What's the difference?
A menstrual period only happens when you're not pregnant in response to ovulation and the uterine lining shedding when the egg has not implanted. Each month, your uterus grows a thick blood-rich lining in preparation for an egg to embed there. If you don't get pregnant that month, you shed this tissue and bleed — that's your period. But once an egg embeds in the uterine lining, hormones tell the blood-rich tissue to stay intact to support the fetus.
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.