Vaginal bleeding during pregnancy is not uncommon, but it's also usually not normal. It can indicate many things, depending on whether it's heavy or light, how long it lasts, what color it is, and at what point in the pregnancy it occurs.
Call your doctor or midwife immediately if you have more than a dime-size spot of blood. Bleeding is a major warning sign of miscarriage, problems with the placenta, or preterm labor. Signs that bleeding is due to one of these serious conditions include heavy bleeding (similar to menstrual bleeding), bleeding in conjunction with cramps and/or fever, or bleeding and passing some tissue.
The color of the blood is also important -- blood that's bright red is usually more worrisome than blood that's brownish.
However, there are also less serious or benign causes for spotting and bleeding (some possible causes include recent fertilization, a recent Pap smear or pelvic exam, having sex during the second or third trimester, or an infection). Many women who spot go on to deliver healthy babies. However, even if you suspect that your spotting is not serious, you should still call your doctor. --Kate Kelly