First, know that both these problems are pretty rare — placental abruption occurs in about one percent of pregnancies, and placenta previa in only half a percent.
Placental abruption occurs when the placenta detaches from the wall, and usually happens in the last trimester or during labor. This may decrease the oxygen your fetus is able to take in and put his health in danger. Vaginal bleeding and stomach pain are both signs of abruption, so call your doctor right away if you notice either one. (Don’t start panicking, though — these same symptoms can also be caused by many minor conditions.) If you’re over 35, have already had one or more children, have experienced a previous abruption or have sickle cell anemia,you’re at particular risk for abruption. High blood pressure, stomach injury,smoking and cocaine use have also been linked to the condition. (Addressing these issues certainly won't hurt the overall health of you and your baby, either!)
Placenta previa, which occurs when the placenta lies low in the uterus and covers the cervix partially or completely, is also often signaled by vaginal bleeding(though no pain). You’re more susceptible if you’ve already had a child, are carrying multiples, or have gone through a c-section or other uterine surgery.
American College of Obstetrics and Gynecologists. Your pregnancy and birth. 4th ed. Washington, DC: ACOG; 2005.