Having a heavy period generally means that a woman is bleeding enough to completely saturate a tampon or pad each hour for two or three hours in a row. It is not possible to have a heavy period and be pregnant. Experiencing light vaginal bleeding, however, is fairly common during pregnancy, so getting what appears to be a very light or short period doesn't always mean that you're not pregnant. Heavy bleeding in women who are pregnant may be a sign of a miscarriage or an ectopic pregnancy.
A few days into your menstrual cycle, estrogen stimulates the lining of your uterus to become thicker. If fertilization of an egg occurs after ovulation, the fertilized egg will move to the uterus and be nourished by that thick lining. If the egg is not fertilized, hormones stimulate your uterus to shed the lining at the end of your cycle. This shedding is your period. Experiencing a heavy or even a normal period means that you cannot be pregnant, since your uterus has shed its lining instead of maintaining it to sustain a fertilized egg.
While you cannot experience a heavy period and be pregnant, it is not uncommon to have light bleeding or spotting during the first few months of pregnancy. In rare cases, this bleeding may seem like a period because it occurs at the same time that your period normally would appear. In such cases, however, the bleeding will be much lighter and of a shorter duration than a normal period.
Light bleeding also can be an indication of an impending miscarriage. If you experience light bleeding that becomes heavy, you may be having a miscarriage. This can occur even before you know that you are pregnant.
Vaginal bleeding is one of the symptoms of an ectopic pregnancy, a pregnancy in which a fertilized egg implants itself somewhere other than the uterus. Bleeding as a result of an ectopic pregnancy is usually light, but it's possible for it to be heavy, too. An ectopic pregnancy can be life-threatening if not promptly treated.
Consult your doctor if you are pregnant and experience any light bleeding. Although spotting may be normal, it also could be a sign of a more serious problem. If spotting turns into heavy bleeding, seek medical attention immediately.