According to the website Med Guidance, there is no reason to believe definitively that a woman is pregnant if her period lasted only three days. There are many reasons why a woman's period would last only three days. A three-day period is not cause to believe a woman is pregnant unless other conditions are met.
The lengths of women's menstrual periods vary. Many women experience periods that are as long as seven full days, while others may have a normal period of only three days. A period can even last only one or two days, in which case some may be concerned about the possibility of pregnancy.
According to Baby Zone, irregularities in a woman's menstrual cycle are sometimes the result of hormone fluctuations. Other reasons for menstrual irregularity include illness and stress. Some women report being "regularly irregular." Symptoms of pregnancy often mimic the symptoms of an irregular menstrual cycle, which is why many women grow concerned when they notice changes in their period.
According to WebMD, early symptoms of pregnancy include spotting or bleeding for a short period of time, often mimicking a period. Thus, if a woman experiences a three-day period, she may actually be spotting. Other symptoms of pregnancy include breast changes and cramping.