This doesn't usually occur until close to the end of your second trimester or later when the cervix thins out, stretches and tears the placenta loose. Bleeding with placenta previa may occur without warning and may be extremely heavy. It occurs when the cervix begins to dilate with early labor, and blood escapes. Placenta previa should be suspected when a woman experiences vaginal bleeding during the latter half of pregnancy. The problem cannot be diagnosed with a physical exam because a pelvic examination may cause heavier bleeding.
Doctors use ultrasound to identify placenta previa.
Ultrasound is particularly accurate in the second half of pregnancy as the uterus and placenta get bigger. If you know you have placenta previa, your doctor may tell you not to have a pelvic exam. This is important to remember if you see another doctor or when you go to the hospital. The baby is more likely to be in a breech position with placenta previa. For this reason, and to control bleeding, a Cesarean delivery is almost always performed.