Calculating Pregnancy Due Dates: Naegele’s Rule and Beyond

Accurate pregnancy due date calculation guides every subsequent decision in antenatal care: timing of screening, assessment of fetal growth, and the threshold for inducing delivery if the pregnancy continues beyond term. This guide explains the most useful methods and where each one fits in modern obstetric practice.

Why dating accuracy matters

An incorrect estimated date of delivery (EDD) by even a week can lead to inappropriately early or late delivery, false-positive screening for fetal growth restriction, or missed opportunities for time-sensitive interventions such as antenatal corticosteroids. Modern guidelines therefore prioritise early ultrasound dating over menstrual dates whenever there is a discrepancy.

Naegele’s rule

The classic formula for EDD: take the first day of the last menstrual period (LMP), subtract three months, and add seven days (and one year). This assumes a regular 28-day cycle with ovulation on day 14.

Worked example: LMP 10 February 2026 → subtract 3 months = 10 November 2025 → add 7 days = 17 November → add 1 year = 17 November 2026. EDD: 17 November 2026.

Adjusting for cycle length

For cycles longer or shorter than 28 days, adjust the EDD by the difference. A 35-day cycle adds 7 days to the calculated EDD; a 21-day cycle subtracts 7. Irregular cycles, recent hormonal contraception or breastfeeding all reduce the reliability of LMP-based dating.

Ultrasound dating

First-trimester crown-rump length (CRL) measurement is the gold standard. Most national guidelines recommend redating the pregnancy to the ultrasound estimate when the difference exceeds:

• 5 days at <9 weeks
• 7 days between 9-15+6 weeks
• 10-14 days in the second trimester

Once a pregnancy has been dated by reliable ultrasound, the EDD should not be changed by later scans.

Conception-based dating

For IVF pregnancies, the date of embryo transfer or oocyte retrieval gives the most accurate dating. EDD = transfer date + (266 – days the embryo had developed).

Common pitfalls

• Using the date of a positive pregnancy test instead of LMP.
• Forgetting to add a year when the LMP is in the calendar year before delivery.
• Trusting LMP dates after recent miscarriage, breastfeeding, or oral contraceptive use.
• Re-dating a pregnancy in the third trimester based on biometry — this measures growth, not gestational age.

Use a gestational wheel for speed

Mental arithmetic is fine when you have time, but most clinicians use a gestational wheel for quick LMP-based dating, gestational-age calculations, and timing of screening windows. Our OB Wheel app reproduces the classic paper wheel with adjustments for cycle length and IVF dating.

Key takeaways

• Use Naegele’s rule for a quick LMP-based EDD, then refine with ultrasound.
• First-trimester CRL is the most accurate non-IVF dating method.
• Don’t redate based on third-trimester biometry.
• Adjust for cycle length and confirm reliability of the menstrual history before trusting LMP.