Pregnancy Due Date Calculator
Estimate a due date and gestational age from three methods.
Estimate a due date
Estimate
Estimated due date
Enter a date to see an estimate.
- Gestational age
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- Estimated trimester
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- Dating starts from
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An estimate only. Most births do not happen on the estimated date, and a clinician may revise dating after an ultrasound or examination.
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How to use this calculator
Choose the method that matches the information you have, then enter that date. The last-period method also accepts an average cycle length so the estimate can be shifted for cycles that are not 28 days.
The result reports an estimated due date, an estimated gestational age in completed weeks plus days, and an estimated trimester. Every figure is an estimate rather than a scheduled event.
The three methods
The last-period method adds 280 days to the first day of your last menstrual period. It assumes a regular cycle and an accurately remembered date.
The conception method adds 266 days, because pregnancy dating conventionally starts about two weeks before conception. A conception date is usually itself an estimate unless it is known precisely.
The ultrasound method uses a due date you were already given, and calculates gestational age and trimester from it. Ultrasound dating is generally what a clinician relies on when it differs from a date-based calculation.
What the cycle adjustment is and is not
The cycle-length field shifts the estimate by the difference between your cycle and 28 days. It is arithmetic on a calendar, not a clinical correction, and it cannot account for irregular cycles, variable ovulation timing, or anything observed during an examination.
If your cycles vary substantially, the adjusted date is best read as one point in a range rather than a more precise answer.
Reading the estimate
Actual delivery dates vary widely around any estimate, and a pregnancy that continues past the estimated date is common. Clinical dating can change after an ultrasound or a clinician’s review, and the revised date is the one your care team will work from.
This calculator performs calendar arithmetic only. It does not assess a pregnancy, monitor anything, or replace prenatal care. If you have questions about a pregnancy, dating, symptoms, or your care, contact a qualified healthcare professional.
Frequently asked questions
How is the due date estimated from a last period?
The calculator adds 280 days to the first day of the last menstrual period, the convention often called Naegele's rule. It assumes a regular cycle and that the date entered is accurate, neither of which is true for everyone.
What does the cycle length adjustment do?
It shifts the estimate by the difference between your cycle length and 28 days, so a 35-day cycle moves the date 7 days later. This is a simple calendar adjustment, not a clinical correction, and a clinician may date a pregnancy differently.
Why does the conception method use 266 days?
Pregnancy dating conventionally begins at the last menstrual period, about two weeks before conception. A conception-based estimate therefore uses 280 minus 14 days. Conception dates are usually themselves estimates.
How accurate is an estimated due date?
It is an estimate, not a prediction. Relatively few births occur exactly on the estimated date, and clinical dating is frequently revised after an ultrasound or a review of your history. Treat any single date as an approximate midpoint.
What are gestational age and trimester estimates based on?
Both are measured from the dating anchor, which is the estimated due date minus 280 days, so all three methods produce comparable figures. Trimester boundaries follow a common convention, and clinicians may describe them slightly differently.