Bleeding during pregnancy is not at all uncommon. In fact, over 10 percent of all pregnant women will experience this at some point in their pregnancies, and about 25 percent will have spotting during the early months. Of course, knowing that this is a common occurence doesn't help much if it's happening to you. It can be a frightening development.
If it's more than a light spotting, you should contact your doctor or midwife, especially if the blood is a bright red, or if you're also having stomach pain or cramps.
The Pregnancy Information Guide gives you some valuable information about the different causes of bleeding.
"You might expect your pregnancy hormones to overwhelm your normal hormonal cycle but this doesn't always happen. Some women report bleeding during early pregnancy that strongly resembles having periods during pregnancy. It is actually now thought to be caused by the variation in background hormonal levels that just happen to be greater in some women than in others."