30% of women will spot or have some bleeding during their first trimester. 50% of those will have normal pregnancies. So, the reality is, yes it could be a sign of impending miscarriage, or it could be normal. They don't really know for sure what causes it, but some reasons can be sex during pregnancy (doesn't cause miscarriage, just bleeding sometimes) as your cervix is more sensitive; your body getting rid of old blood, etc. It is always best to call your Dr. and let him/her know.