How due dates are calculated
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. The 280-day figure assumes a 28-day cycle with ovulation on day 14. For cycles shorter or longer than 28 days, the due date shifts accordingly.
Ultrasound dating in the first trimester (10-13 weeks, using crown-rump length) is the most accurate method and supersedes LMP dating when there's a discrepancy of more than 5-7 days. Third-trimester ultrasound dating is unreliable due to normal growth variation between fetuses.
Accuracy of due dates
Only about 5% of babies are born on their exact due date. Around 80% are born within 2 weeks either side. Full term is 39-40 weeks; early term 37-38 weeks; late term 41 weeks; post-term 42+ weeks. Induction is typically offered at 41+0 to 42+0 weeks depending on local guidelines.
Related tools
For gestational weight gain targets, see Gestational Weight Gain. For ovulation timing, see Ovulation Calculator. For menstrual cycle tracking, see Menstrual Cycle Calculator. For newborn assessment at delivery, see APGAR Score.
References
- NICE Guideline NG201. Antenatal care. NICE, 2021.
- ACOG Practice Bulletin No. 700. "Methods for estimating the due date." Obstet Gynecol. 2017;129(5):e150-154.