From the last period
A last period starting January 1 with a typical 28-day cycle gives an estimated due date of October 8.
Pregnancy calculator
This is an estimate for information only. It does not replace prenatal care or dating confirmed by a healthcare professional.
Result
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How it works
Every method here works by finding the equivalent "day one of the last period," even when that is not what you actually entered — an ultrasound estimate, for instance, is converted backward by subtracting the gestational age it reported. From that reference point, Naegele's rule adds 280 days to reach the estimated due date.
The week-by-week table then counts forward from the same reference point, so every method produces a calendar on equal footing.
Formulas used
Due date from last period
Due date = Last period + 280 days + (cycle length − 28)
Due date from conception
Due date = Conception date + 266 days
Worked examples
From an early ultrasound
An ultrasound on February 1 showing 8 weeks 2 days of gestational age implies the same due date as if the last period were about 8 weeks 2 days before February 1.
Assumptions and limits
- A full-term pregnancy is estimated at 280 days (40 weeks) from the last period.
- The last-period method assumes ovulation occurs 14 days before the next period, adjusted for the cycle length entered.
- Milestones are general guidance, not a substitute for prenatal care.
Frequently asked questions
How accurate is a due date?
It is a statistical estimate, not a prediction — only a small percentage of babies arrive exactly on the calculated date. A first-trimester ultrasound is generally considered the most accurate dating method.
Why does cycle length matter for the last-period method?
The standard 280-day rule assumes a 28-day cycle with ovulation around day 14. A longer or shorter average cycle shifts ovulation, and this calculator adjusts for that.
Why would an ultrasound date differ from a last-period date?
Ultrasound measures the size of the fetus directly, which is often considered more reliable than a last-period estimate if cycles are irregular or the last period date is uncertain.
How does the IVF calculation work?
A transferred embryo is already 3 or 5 days past conception, and conception itself corresponds to roughly day 14 of a standard cycle — the calculator works backward through both steps to find the equivalent last-period date.
Updated
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