A lot of women bleed during the first trimester.
I was one of them.
I started
bleeding at 5 weeks and it didn't stop until 10 weeks. I had a subchorionic bleed. It happens because the placenta is developing and attaching itself to the walls. The placenta doesn't become fully used until the beginning of the 2nd trimester, around 12-14 weeks. Until then, it's the yolk sac. Anyways, my baby is now 9 weeks old and is very healthy.