During most normal pregnancies, a woman's natural period goes into remission. However, a woman may have period-like bleeding for the first part of her pregnancy due to a hormone imbalance. This is known as decidual bleeding and is commonly referred to as "breakthrough bleeding." It can take the form of a light period.
Decidual bleeding usually happens at the same time that you would have had your period if not pregnant: for most women, at weeks 4, 8 and 12 of pregnancy.
Decidual bleeding occurs when the hormones that would normally put your period into remission are not yet at a high enough level to do their job. At this time, the ovaries are responsible for hormone production.
Because breakthrough bleeding occurs before the uterus lining and placenta are fully connected, it usually does not last through an entire pregnancy---only the first few months. After the uterus lining and placenta fully join, the placenta takes over hormone production from the ovaries and puts the period into remission.
Decidual bleeding is not a sign of a problem with the pregnancy; it is simply a sign that the ovaries are not producing sufficient hormones to put your period into remission. This should resolve after the third month of pregnancy, when the placenta takes over hormone production.
According to the Mayo Clinic, most women who have decidual bleeding or other early-pregnancy vaginal bleeding have healthy, full-term babies.