Bleeding in the uterus (subchronic Hemorrhage) does anyone else have this?

Im 13 weeks 2 days pregnant with twins, I didn't know I was pregnag until my doctor did a pregnancy test & it was positive, I was around 7 1/2 weeks. A few days later I had some bleeding so thought I was having a miscarriage. I've had one miscarriage, then went on to have two kids after. They said I wasn't having a miscarriage & the ultrasound showed I was expecting twins. They found a blood clot in my uterus around 5-6cm big, it has gone down to 3 cm, & was told they usually resolve themselves. Also that it could cause premature labor, & carrying twins is risk for premature labor also. Just curious of anyone else knows anything about this?