A last period starting 10 weeks ago, 28-day cycle
The estimated due date is 280 days after that date, with the current pregnancy at about 10 weeks of gestational age.
Estimated due date calculator (Naegele’s rule)
For general informational and healthcare professional reference only. This is a statistical estimate, not a diagnosis or a medical prediction. Pregnancy dating and care decisions should be confirmed and managed by a qualified healthcare provider, typically with an ultrasound.
Result
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How it works
Naegele’s rule estimates a due date as 280 days (40 weeks) after the first day of the last menstrual period, based on a theoretical 28-day cycle with ovulation on day 14. For a longer or shorter average cycle, the estimate shifts by the difference from 28 days, since ovulation (and so conception) would have happened correspondingly later or earlier.
When the ovulation date is actually known — for instance from fertility tracking — the due date is estimated as 266 days (38 weeks) after ovulation instead, which sidesteps the cycle-length assumption entirely and is generally considered more precise when available.
Gestational age is conventionally counted from the last menstrual period, not from conception, which is why it is already about two weeks at the moment of ovulation and conception in a standard 28-day cycle.
Formulas used
Naegele’s rule (from LMP)
Due date = LMP + 280 days + (cycle length − 28 days)
From ovulation date
Due date = ovulation date + 266 days
Worked examples
A known ovulation date from cycle tracking
The due date is estimated directly as 266 days later, without needing to assume a cycle length.
Assumptions and limits
- Naegele’s rule assumes ovulation occurs 14 days before the next expected period, adjusted here for a non-28-day average cycle.
- This does not account for irregular cycles, assisted reproduction timelines with a known transfer date, or a clinically confirmed ultrasound dating — any of which should take precedence when available.
Frequently asked questions
Why isn’t the actual birth date usually the estimated due date?
A full-term pregnancy spans a range, conventionally 37 to 42 weeks, and Naegele’s rule assumes a textbook 28-day cycle with ovulation exactly on day 14 — real cycles and implantation timing vary. The estimate is a useful planning midpoint, not a firm prediction.
Which method is more accurate, LMP or ovulation date?
When the ovulation date is reliably known (from tracking basal temperature, ovulation tests or fertility treatment timing), it removes the assumption of a standard 28-day cycle and is generally considered the more precise of the two. An early first-trimester ultrasound is more precise than either and is what clinicians typically use to confirm or adjust the estimate.
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