“Period” while Pregnant

So I’ve heard multiple stories, including my own grandmother, who have said they didn’t know they were pregnant until about 4-5 months along because they continued to bleed every month around the same time they would get their period. The same amount, blood clots, cramps, and all.

How many women has this happened to?

Do you personally know a friend/family member/etc that this has happened to?

Does anyone know how it’s possible?

If you have stories, feel free to share them.

I understand that pregnant women don’t get their periods, but from the stories I’ve heard, it sounds just like one. Same blood consistency, clots, full 5-10 days, cramps and all.