It is common to have some light vaginal bleeding sometime in the first 12 weeks of pregnancy.

http://www.patient.co.uk/health/miscarriage-and-bleeding-in-early-pregnancy

What is a threatened miscarriage?

It is common to have some light vaginal bleeding sometime in the first 12 weeks of pregnancy. This does not always mean that you are going to miscarry. Often the bleeding settles and the growing baby is healthy. This is called a threatened miscarriage. You do not usually have pain with a threatened miscarriage. If the pregnancy continues, there is no harm done to the baby.

In some cases, a threatened miscarriage progresses to a miscarriage.