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A due date is typically calculated by adding 280 days (40 weeks) to the first day of the woman’s last menstrual period (LMP). This method assumes a 28-day menstrual cycle with ovulation occurring on day 14. This is known as Naegele’s Rule. However, due dates are estimates, and only about 5% of babies are born on their exact due date. Most healthcare providers consider a pregnancy full-term between 39 weeks and 40 weeks 6 days.

Due date calculations are estimates and not exact dates. The accuracy depends on several factors including the regularity of the menstrual cycle, the accuracy of the LMP date, and whether ultrasound measurements are used. Early ultrasounds (before 13 weeks) are generally more accurate for dating pregnancies than later ultrasounds. Even with the most accurate calculations, only about 70% of babies are born within 10 days of their due date.

If you don’t know the date of your last period, your healthcare provider can use an early ultrasound to estimate your due date. Ultrasounds in the first trimester are the most accurate method for dating a pregnancy when the LMP is unknown. The size of the fetus, particularly the crown-rump length (CRL), can be measured to estimate gestational age within 5-7 days of accuracy.

Yes, due dates can change during pregnancy, especially if there’s a significant discrepancy between the LMP-based due date and ultrasound measurements. If an early ultrasound shows a difference of more than 5-7 days from the LMP-based due date, your healthcare provider may adjust your due date. Later in pregnancy, if the baby is measuring significantly larger or smaller than expected, your due date might be adjusted, though changes are less common in the third trimester.

The chances of delivering exactly on your due date are quite low—only about 5%. Most babies are born between 37 and 42 weeks of pregnancy, which is considered the full-term period. About 60% of babies are born within a week of their due date (either before or after), and about 90% are born within two weeks of their due date. Healthcare providers often give a “due window” rather than a single date to manage expectations.