Pregnancy Due Date Calculator

Pregnancy Due Date Calculator for Obstetrics & Gynecology. An estimated due date is a dating reference, not a prediction of the exact day of birth. Gestational age counts obstetric weeks, not weeks since fertilization. Negative ages and a future LMP relative to the assessment date produce no result. A date beyond the due date can still produce a positive gestational age; days are never made positive with an absolute-value operation. The displayed assessment date makes historical calculations reproducible.

For shared guidance on choosing a tool and documenting results, read the clinical calculator workflow.

How this calculator works

The base menstrual estimate is LMP plus 280 calendar days. This implementation adds cycle length minus 28 as an explicitly approximate regular-cycle adjustment. It then calculates gestational days as 280 minus the number of days from the assessment date to that adjusted due date. Completed weeks are the integer quotient after division by seven, with the remainder shown as days. This keeps the displayed gestational age consistent with the displayed due date. Calendar-day arithmetic avoids time-of-day rounding and daylight-saving shifts. Dates are displayed in unambiguous year-month-day order.

When to use this calculator

Use an LMP estimate when a regular-cycle history is available and a provisional date is needed. Confirm the LMP and assessment dates before interpreting the result. ACOG Committee Opinion 700 places dating in the context of early ultrasound and the best obstetric estimate. A recorded due date based on appropriate ultrasound or assisted reproduction should not be overwritten by a new menstrual calculation. The cycle adjustment assumes later or earlier ovulation follows the cycle-length difference; this assumption is not an ultrasound finding.

Inputs used

  • The first calendar day of a recalled last menstrual period, in YYYY-MM-DD form.
  • Usual regular cycle length as a whole number of days; supported range 21-35.
  • Assessment calendar date for the gestational-age estimate, initially the local current date.

Clinical interpretation

An estimated due date is a dating reference, not a prediction of the exact day of birth. Gestational age counts obstetric weeks, not weeks since fertilization. Negative ages and a future LMP relative to the assessment date produce no result. A date beyond the due date can still produce a positive gestational age; days are never made positive with an absolute-value operation. The displayed assessment date makes historical calculations reproducible.

Worked example

For LMP 2026-01-01, a 28-day cycle gives an estimated due date of 2026-10-08. On 2026-03-12, 70 calendar days have elapsed, so the estimate is 10 weeks and 0 days. Changing only the cycle to 30 days moves the due date to 2026-10-10 and the corresponding age to 9 weeks and 5 days. The unadjusted 70 days since LMP remains visible. On the estimated due date, this calculation always shows 40 weeks and 0 days.

Limitations and safety notes

Irregular cycles, uncertain bleeding dates, variable ovulation and recent hormonal contraception can undermine menstrual dating. This tool does not reconcile ultrasound discrepancies, date an embryo transfer or decide when delivery is indicated. The supported 21-35-day cycle and 0-45-week age ranges are software scope limits, not a definition of normality or a clinical validation population. A blank, fractional or malformed cycle is rejected rather than silently replaced by 28. Local current-date initialization is separate from timezone-independent calendar arithmetic.

Frequently asked questions

Why does changing cycle length also change gestational age?

The adjusted due date is the reference for the displayed age. Adjusting only the due date would leave two inconsistent dates on the same screen. Unadjusted elapsed LMP days are shown separately so the assumption remains visible.

References

  • https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
  • https://www.acog.org/practice-management/health-it-and-clinical-informatics/revitalize-obstetrics-data-definitions

Editorial review and citation methodology

Maintained by the Quick Medical Calculator Editorial Team. Content record date: September 9, 2026. A content date is not evidence of independent clinical review. Individual clinical sign-off is not recorded on this page.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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