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Due dates are also calculated based on the baby's body length during an ultrasound between eight and 12 weeks of pregnancy, according to Dr. Sachchidananda Maiti, England-based obstetrician and gynecologist.
As the pregnancy progresses, the accuracy of an ultrasound for predicting due dates decreases. Between 18 and 28 weeks of gestation, the margin of error increases to plus or minus two weeks. After 28 weeks, the ultrasound may be off by three weeks or more in predicting a due date. Thus, later in pregnancy, ultrasound measurements are more valuable for evaluating the baby's growth over time (compared with earlier measurements) than they are for predicting a due date.