The results of an ultrasound are compared to what is expected for the gestational age of a pregnancy. The gestational age is calculated by the number of weeks since the last menstrual period; however, this method generally assumes a 28-day cycle with ovulation occurring on the 14th day. Many women have shorter or longer cycles and do not ovulate on the 14th day -- and this could affect what an ultrasound should show for development of the pregnancy.
For example, if a woman has a 35-day menstrual cycle, she most likely ovulates around the 21st day of her menstrual cycle (because ovulation usually occurs two weeks before the menstrual period would begin). If the woman became pregnant and had an ultrasound scan six weeks from her last menstrual period date, her normally developing pregnancy would measure with a gestational age of five weeks, because the gestational age dating system assumes that she would have ovulated a week earlier than she did. If the woman did not know that she ovulated on the 21st day of her cycle, she might worry unnecessarily that she was having a miscarriage if she had an early ultrasound that showed only five weeks of development in her pregnancy when she was technically "six weeks pregnant."
Similarly, not everyone meticulously tracks the start of the menstrual period. If a woman cannot remember when her menstrual period started and guesses the wrong day, even if she does have a typical 28-day cycle, this could also change the expected results of an ultrasound scan.
In early pregnancy, the gestational sac and embryo change drastically every day, so being off even by a few days with dating can make a difference in whether or not the ultrasound should detect a heartbeat.