Why have i been bleeding during pregnancy?

Bleeding; pregnant: Several possibilities: the pregnancy may be miscarrying; the placenta may have partly detached (with possibility of complete detachment; so see OB asap! [abruptio placentae]; the placenta may partly/completely overlay the cervical opening [placenta previa], a condition which also needs urgent attention of your ob.

See a doctor: If you're bleeding while pregnant it's possible that you may be at risk of a miscarriage. While there might not be much a doctor can do to prevent miscarriage if it's actively occurring, the doctor can give a sense of likelihood and rule out other issues/contributing problems, such as infections.