Due Date Calculation Methods: LMP vs. Ultrasound — Which Is More Accurate?
Naegele’s Rule: The LMP Method
Naegele’s rule: add 7 days to the first day of your last menstrual period, then add 9 months (or subtract 3 months). This assumes a 28-day cycle with ovulation on day 14. For cycles longer than 28 days, add the extra days. For 35-day cycles, add 7 days to the estimated due date.
Ultrasound Dating: Crown-Rump Length
An ultrasound at 8–12 weeks measures crown-rump length (CRL) — the most accurate dating method. At this stage, gestational age can be estimated to within ±5 days. After 12 weeks, accuracy decreases because fetal size variation increases. First-trimester ultrasound changes the due date for about 15–20% of pregnancies.
Which Method to Trust
First-trimester ultrasound is more accurate than LMP dating when cycle length is irregular, when LMP is uncertain, or when there’s a discrepancy of more than 5 days between LMP and ultrasound dating. Pregnancy Calculator uses Naegele’s rule as the standard with cycle length adjustment.
What ‘Due Date’ Actually Means
Only about 5% of babies are born on their exact due date. The ‘normal’ range is 38–42 weeks. The due date is week 40 — medical management typically begins if labor hasn’t started by 41–42 weeks. Think of it as a due window, not a due date.
Frequently Asked Questions
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