Pregnancy bleeding

My best friend is 12 weeks pregnant and has been bleeding off and on the entire time. The baby is growing great and everything looks fine but doctors are concerned and don't have any answers in regards to her bleeding. She already suffered one miscarriage when she was five months pregnant with her first pregnancy and then suffered through three years of infertility so she is really freaking out. Has any one else ever heard of this or have any experience with bleeding throughout pregnancy? And tips, advice, or information? Thanks in advance.