Pregnancy Due Date Calculator
Last updated: October 15, 2026
Estimate a pregnancy due date from the first day of the last menstrual period.
Estimated due date
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- Estimated pregnancy length: —
- Estimated conception date: —
On this page
Quick summary: Estimate a pregnancy due date from the first day of the last menstrual period.
Read the complete guide
Updated reference sections in this guide are in English.
Guide language: English. Updated October 15, 2026.
What this calculator does
Estimate a due date by adding 280 calendar days to the first day of the last menstrual period, using a simple 28-day-cycle assumption.
How to use the calculator
Use the inputs and units named in this form. Select Calculate after changing the values.
How the calculation works
The tool adds 280 days, or 40 weeks, to the entered last-period start date. Its conception-date illustration adds 14 days to that same date. The form has no cycle-length, ovulation-date or ultrasound input, so it cannot adjust for those factors.
Worked example
A last-period start date of January 1, 2026 gives an estimated due date of October 8, 2026. The conception-date illustration is January 15. The 40-week output describes the model's assumed interval, not how far along a pregnancy is today.
How to interpret the result
Compare the result with the assumptions and units shown above. The calculation uses the values entered into this page.
Assumptions and limitations
This is a calendar estimate. Cycle length and uncertain period dates can affect it. NHS guidance explains that an antenatal scan can estimate pregnancy timing more accurately. The conception illustration does not establish the actual day of conception or any legal conclusion.
Common mistakes
Check the units and time period before calculating. An error message means the result cannot be calculated from the current inputs.
Privacy
The calculation runs in your browser. This form needs the numerical inputs shown, not names, account numbers or passwords.
Frequently asked questions
Does this tool adjust for a longer or shorter cycle?
No. The current form uses the simple 28-day-cycle assumption.
Does 40 weeks mean I am currently 40 weeks pregnant?
No. It is the interval added to the entered last-period date.
Can this replace a clinician's due-date assessment?
No. Use the timing established during antenatal care.