Bleeding during pregnancy is common, especially during the first trimester , and usually it's no cause for alarm. But because bleeding can sometimes be a sign of something serious, it's important to know the possible causes, and get checked out by your doctor to make sure you and your baby are healthy. Bleeding in the First Trimester About 20% of women have some bleeding during the first 12 weeks of pregnancy . Possible causes of first trimester bleeding include: Implantation bleeding . You may experience some normal spotting within the first six to 12 days after you conceive as the fertilized egg implants itself in the lining of the uterus. Some women don't realize they are pregnant because they mistake this bleeding for a light period. Usually the bleeding is very light and lasts from a few hours to a few days. Miscarriage. Because miscarriage is most common during the first 12 weeks of pregnancy, it tends to be one of the biggest concerns with first tr