If boyfriend ejaculate in vagina with period will i get pregnant?

Yes, you may!: Most are not effective at using rhythm method. That's why it's important that you use at least one form of birth control each & every time you have sex, unless you also want children. This is typically a hormonal method. Preferably, you'll also use (well, make him wear) a condom as your 2nd form of birth control, as this will help minimize your risk of std or sexually transmitted diseases.

Probably not, but...: No guarantees. If your menstrual cycles are mostly regular, then intercourse particularly early during your period, is unlikely to cause pregnancy. Later on during your period, and especially if you bleed 6-7 days or longer, it becomes much less safe. If your cycles are unpredictable as well, you could be at considerable risk to get pregnant this way.

You could: Pregnancy is known to have occurred from sex on any and all days of the period. If sex occurred within the past 72 hours, take morning after pill. If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD.