There are a few reasons why you could have spotting or bleeding during pregnancy; some are serious, and some are not, so it is better to get it checked out to be on the safe side. Sometimes it can be because conception took place close to the time of the next period; this has been known to lead to confusion over due dates, so the gestational age of the baby should be determined by an ultra sound.
Sometimes an ectopic pregnancy can cause cramping and bleeding during an early pregnancy. This means that the embryo has implanted itself outside of the uterus, usually in a fallopian tube, and it can be life threatening, so it is vital that medical attention is sought.
It may be hard to believe, but it is a fact that only about a quarter of all pregnancies go full term. Many end before the woman is even aware that she is pregnant, but if there has been a positive test and bleeding begins, it is possible that the woman is having a miscarriage. Unfortunately, although medical attention is important to make sure that there are no complications, there is little that can be done to prevent a miscarriage in the first trimester (12 weeks).
Bleeding later on in the pregnancy could be an indication of placental problems, either with placenta praevia, where the placenta covers the cervix, or placenta abruption, where it comes away from the uterine wall. Either instance could be dangerous if untreated, so medical help is needed immediately.