When to use this calculator
Estimate a pregnancy due date from the last menstrual period or a known conception date. An estimated date, not a predicted delivery day or confirmation of pregnancy. The last-period method assumes a regular 28-day cycle. Irregular cycles and uncertain dates reduce accuracy. Clinician assessment and early ultrasound may establish a different date; use that clinical date for care.
Explore the pregnancy & cycle calendars calculators for other questions in this topic. For a related calculation, use the BMI Calculator: Calculate adult body mass index from height and weight.
Choose the date type you actually know
The first day of the last period and the conception date are different starting points. Last-period dating adds 280 days and assumes a regular 28-day cycle. Conception dating adds 266 days, but a date of intercourse is not necessarily a known conception date. A calendar estimate cannot establish which date is clinically correct.
PUTTING IT INTO PRACTICE
Pregnancy Due Date: a worked example
The values below are an illustrative example, not a recommendation. Replace them with measurements or assumptions that match your question.
| Input | Example value |
|---|---|
| First day of last period, or known conception date | 2026-01-01 |
| Dating method | lmp |
| Result | 2026-10-08 |
- Check the input labels and enter the example values above.
- Apply the relationship: Estimated due date = first day of last period + 280 days, or known conception date + 266 days..
- The result is 2026-10-08 , rounded for display.
How to interpret your result
An estimated date, not a predicted delivery day or confirmation of pregnancy. The last-period method assumes a regular 28-day cycle. Irregular cycles and uncertain dates reduce accuracy. Clinician assessment and early ultrasound may establish a different date; use that clinical date for care.
These calendars cannot confirm pregnancy, identify all fertile days or diagnose cycle changes. They are unsuitable for contraception. A clinician-established pregnancy date takes precedence over a calendar estimate.
Frequently asked questions
Can this confirm pregnancy?
No. Calendar dates cannot confirm pregnancy or replace a pregnancy test and clinical assessment.
Should I change a clinician-established due date?
No. Discuss differences with your maternity care team rather than using this estimate to change your care plan.
Why might an ultrasound due date differ?
Uncertain period dates, variable ovulation and cycle patterns can affect calendar dating. A maternity professional can assess early ultrasound and other information; follow the date established for your care.
Can a cycle calculator identify safe days for contraception?
No. Ovulation can shift and a calendar cannot reliably identify all fertile days. Use qualified advice about contraception.
When is the pregnancy due date calculator useful?
Estimate a pregnancy due date from the last menstrual period or a known conception date. An estimated date, not a predicted delivery day or confirmation of pregnancy. The last-period method assumes a regular 28-day cycle. Irregular cycles and uncertain dates reduce accuracy. Clinician assessment and early ultrasound may establish a different date; use that clinical date for care.
How do I use the pregnancy due date calculator?
Enter first day of last period, or known conception date, dating method. The result updates automatically. Use the units shown beside each input; the example values illustrate the method.
Which formula does this calculator use?
Estimated due date = first day of last period + 280 days, or known conception date + 266 days..
What should I check before using the result?
An estimated date, not a predicted delivery day or confirmation of pregnancy. The last-period method assumes a regular 28-day cycle. Irregular cycles and uncertain dates reduce accuracy. Clinician assessment and early ultrasound may establish a different date; use that clinical date for care.
How are results rounded?
Scalar numbers display up to six decimal places, with scientific notation for very large or small values. Text, fraction, date and schedule outputs use the format described on the tool. Intermediate calculations use JavaScript floating-point arithmetic. Keep sensible precision for your measurements.
Further reading
For background on this topic, see ACOG: methods for estimating the due date ↗. This is a topic reference, not an external review or endorsement of this tool.
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