Understanding Large Perigestational Hemorrhage: Causes & Management

Question
I am 26, I have a 20 month old boy and am 12 wks pregnant.  my due date is july 25th.  A few days ago I had a "gush" of red blood come out all of a sudden, with mild cramping and some more heavy bleeding later throughout the day.  After seeing the doctor the next day, they found that my perigestational blood clot had grown from 2 cm to 9.3 cm in 4 weeks.  This seems large as they stated and told me to take it easy and no exercise.  Im wondering what causes this to grow and if it could still grow?  Can something this big resolve itself inside without having any more major bleeding episodes?  I am moving this weekend out of state and worry what to do if the bleed happens again or if it doesn't go down. Thank you for your help.  

Answer
It is common at 12 weeks to have a "subchorionic blood clot" which is a sequestration of blood within the folds of the developing placenta. It usually does not cause any harm to the pregnancy or to the baby and usually dissipates on its own within a few weeks. Avoid straining, heavy lifting, exercise, and sexual activity until the bleeding completely disappears. If you are moving, try not to do any lifting or carrying of heavy things. If you notice bleeding agaiin, just get into bed and stay there until it subsides. Then you have to have another sonogram to assess what is going on. Don't worry. This is quite common.