Ultrasounds are most effective for calculating a woman's delivery date when performed between the eighth and 18th weeks of her pregnancy, according to the American Pregnancy Association. Predictions made with ultrasounds become less accurate as a woman's pregnancy progresses.
Doctors frequently use ultrasounds in conjunction with mathematical calculations from the date of a woman's last period to calculate an accurate due date, notes the APA. Even for ultrasounds performed within the early weeks of a woman's pregnancy, there remains a margin of error of approximately 1.2 weeks, according to Krissi Danielsson for About.com. Some women are unable to recall the date of their last period or do not experience a regular menstrual cycle, in which case it is common for doctors to rely solely on the estimation provided by an ultrasound to calculate a woman's due date.
Doctors continue to perform ultrasound scans during the length of a woman's pregnancy to examine the baby for developmental changes and to make sure the fetus is growing according to the expected milestones. If a doctor sees that the baby is behind or ahead of schedule, it is possible to revise the original due date by a few days, notes Danielsson. This may not be necessary, because the margin of error for late-term ultrasounds grows to two or three weeks.