Light, brief (lasting just a day or two) spotting can actually be a sign of early pregnancy. It’s known as “implantation bleeding,” and here’s what it means: A few days after conception, that little fertilized (yay!) egg starts digging into the wall of your uterus and getting ready to grow. Since the uterine lining is nice and rich with blood, some women spot a bit at this point. Implantation bleeding is totally normal and not a cause for concern. A pregnancy test and doctor's visit is in order, though, to make sure implantation bleeding is truly the culprit.
If you find yourself bleeding heavily or for more than a couple of days, or if you bleed during a later stage of your pregnancy, call your doctor right away. This could be a sign of miscarriage or an ectopic (often called “tubal”) pregnancy.