When implantation bleeding occurs at the beginning of pregnancy, this may be mistaken for a short, two-day period, according to MedGuidance.com. Occurring when a fertilized egg attaches to the uterine wall, implantation bleeding lasts one or two days, can be accompanied by light cramping and is typically pinkish or brownish in color. If pregnancy is not possible or ruled out, fluctuating estrogen levels are often to blame for short periods.
According to MedGuidance.com, rigorous exercise, physical or emotional stress, eating disorders and weight loss or gain can all cause changes in estrogen levels that affect the length of the menstrual cycle. Other possible reasons for a two-day period have to do with the use of oral, injected or intrauterine contraceptives that all create hormonal changes in the body, which can result in a shorter period.
A three- to five-day period is most common; however, in many women it is normal to have a period as short as two days and as long as seven days, according to the Cleveland Clinic. Short periods are not typically a cause for concern, but MedGuidance.com recommends visiting a doctor if changes in the menstrual cycle persist or cause concern. If pregnancy is suspected and a short period follows, take a pregnancy test to confirm or rule out pregnancy.