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most common cause of bleeding during this period of gestation is low lying placenta .Please get your ultrasound repeated from good centre & get it confirmed whether the placenta is low lying or not.placenta should be 5cm higher from internal os of cervix
during medium filled urinary bladder
.Polyp or cyst
rarely cause bleeding with clots.trans-vaginal ultrasound
can give better information.
Wish you good health