Bleeding in the first trimester is very common. It occurs in about 1 in 4 women and is, most of the time, caused by a small blood clot between the amniotic sac and the uterus called a subchorionic hemorrhage or subchorionic hematoma.
The blood clot comes from blood vessels in the uterine wall that are severed during the implantation of the embryo or early placenta.
If you experience bleeding and have a subchorionic hemorrhage, your health care provider may put you on bedrest. If the blood clot is small it can often be reabsorbed by the body.