Bleeding during pregnancy is a warning sign of an abnormal pregnancy. Regardless of whether the amount of bleeding is large or small, pregnant mothers should be vigilant and seek medical attention as soon as possible. Here are 7 possible causes of bleeding during pregnancy to give you an overview. In the first trimester, abnormal vaginal bleeding is a common problem, affecting about 1/4 of pregnant women; in the second and third trimesters, there are still some risk factors that can cause abnormal bleeding. The causes and conditions of bleeding during pregnancy vary from person to person.