If the birth control is successful, the bleeding will clear up in less than a week. However, birth control is only a treatment method and not every woman who is on birth control will be able to keep the fetus successfully. This is because some of the fetuses themselves do not develop normally and experience preterm miscarriage, which eventually develops into inevitable miscarriage and a method of natural elimination. Therefore, if there is black vaginal bleeding in early pregnancy and there is still vaginal bleeding after a week of treatment by birth control, further examination is needed. Sometimes having cervical polyps or chronic cervical inflammation can also result in black blood flow, which can also be caused by refractory miscarriage, and necessary tests can help troubleshoot the cause of vaginal bleeding.