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Q: What causes bleeding during pregnancy?

Bleeding in pregnancy is more common than you might think, especially in the first trimester. In some cases it's nothing to worry about, while in others it can be a sign of a problem.

Around 20 percent of women experience some bleeding or spotting in the first trimester. Common causes include implantation bleeding, which occurs as the fertilized egg implants itself in the lining of the uterus around 10 days after conception, and might look like a very light period. There can be some bleeding between seven to 10 weeks because of a shift in the production of the hormone progesterone from the ovary to the placenta and this is usually not serious. Because pregnancy causes extra blood to flow to the cervix, some women might notice light bleeding after sex or a Pap test. More serious causes of early pregnancy bleeding include infection, ectopic pregnancy, or miscarriage.

Bleeding in the second and third trimester can be more cause for concern. Possible causes include miscarriage, a premature opening of the cervix, problems with the placenta, or preterm labor.

Near the end of pregnancy, it's common to experience light bleeding, and this may be a sign that labor is about to begin.

In general, you should contact your doctor if you experience any bleeding during pregnancy. During the first trimester, you can wait until your next prenatal visit to report any spotting or light bleeding that lasts less than a day. Later on, you should call your doctor the same day if you notice even light bleeding, and you should call your doctor immediately if you experience heavy bleeding at any point in your pregnancy or if the bleeding is associated with any pain.