Hi, Blood clots in the uterus are mostly not harmful and are generally reabsorbed as the pregnancy is progressing. There could have been an uterine bleeding at some point, either due to a ruptured blood vessel or as a result of the placenta being pulled away from the uterine wall (placental abruption). This could have lead to the brownish discharge and scan showing presence of a blood clot. They result from coagulated blood cells and proteins.
To be on the safer side avoid intercourse, strenuous exercises, strain etc., and take rest. Blood clots are rarely harmful, usually follow up sans show blood clots resolved automatically.
Wishing you good luck for your pregnancy.
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