How Pregnancy Due Dates Are Calculated
Naegele’s rule, ultrasound dating and why only 4 percent of babies arrive on the day.
Counting from the last period
Pregnancy is dated from the first day of the last menstrual period and lasts about 40 weeks, or 280 days. Naegele’s rule adds one year, subtracts three months and adds seven days.
Ultrasound dating
An early ultrasound measuring the baby is more accurate than dates if cycles are irregular. Doctors may adjust the due date if the two differ by more than a week.
Due dates are estimates
Only about 4 percent of babies are born on their due date. Full term is 39 to 40 weeks 6 days, and early term and late term have specific definitions.
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Only about 4% of babies arrive on their due date; most are born within two weeks either side. An early ultrasound gives the best dating.
Educational content only. This article does not replace advice from a qualified health professional. Medical disclaimer.