Can you get pregnant if you have sex when you're on your period?
Yes! I mean aside from the factors of anatomy like you don't have a uterus and sexual preference, like you don't have sex with semen producers, or developmental reasons like you're pre-eggs or post-eggs, and then the medical complications of things like pelvic inflammatory disease, or if you've got effective birth control, then yes, you can get pregnant on your period.

Menstruation is not a pregnancy grace period, it's just a period. 

[Intro]

As I've mentioned here in this video, also posted in the description, a period, also known as menstruation, is part of a cycle. Pregnancy happens during the cycle when a sperm-egg combo nestles into the uterine lining called the endometrium.

In order for this to happen, there must be an unexpired egg in the Fallopian tube; they can live for 12 to 24 hours. During that time, sperm needs to be there waiting, or on its way.

That sperm also needs to be capable of fertilizing the egg. If it does, the fertilized egg has to find its way to the uterus, and then implant successfully in the lining.

That's conception!

It would seem like getting pregnant on your period isn't possible because there isn't an endometrium to nestle into, and everything is being flushed out.

Yeah, wouldn't the egg and sperm be evacuated too?

Well, yes, UNLESS...
The egg-sperm combo, a blastocyst, is at the tail end of the bleeding.

You're still experiencing a period, but the blastocyst is above it and able to embed in the reforming endometrial wall.

On average, a menstrual cycle is 28 days. If the period is the first week, one could conceivably have sex during the bleeding and get pregnant.

How? Well, provided sperm were ejaculated, then they would travel the Fallopian tubes to meet the egg. Since they can live there in the Fallopian paradise for up to 5 days, it is possible for viable sperm to be there when an egg is released at ovulation between days 10 and 16 of the 28 days.

I should know, I am this conception. My mother tried for 14 months to conceive at all other times of the month, and then, when she had period sex, she succeeded.
 
So if this happens, you have a looong period and ovulate during or shortly after the bleeding, you can get pregnant on your period.

Or, if you have a super short cycle, where ovulation would happen much closer to the front end... You can get pregnant from having sex during your period!

There's also: you think the blood is menstrual but it's really blood from ovulation. In this case, you're getting pregnant on your "period", what you think is your period really isn't your period, and then you go ahead with sex when...in actuality it could be your most fertile time!

Here's a related anthropology lesson.

Many of the patients in fertility clinics are Orthodox Jews abiding by Leviticus 15:19 in the law of the Torah called niddah.

"And if a woman have issue, and her issue in her flesh be blood, she shall be put apart seven days: and whosoever touchest her shall be unclean until the even."

Meaning that during her period and the seven days after it, her partner may not have sex with her, and in more strict cases, may not touch or sleep with her.

Which depending on the unique menstrual cycle could equate to missing the fertile window.    

There are hundreds of menstruation-related beliefs, customs, and taboo. I just hope to make it clear that the one about not being able to not get pregnant on your period is false. Stay curious.