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Due Date Calculator

Calculate pregnancy due date from last period, conception date, or dating scan.

Updated

What is the Due Date Calculator?

The Due Date Calculator estimates an expected delivery date from the first day of your last menstrual period, a known conception date, or an early dating scan. It also shows gestational age, trimester, and a practical due-date window so the result is easier to interpret in real life.

This tool is designed for fast, client-side use with no sign-up, no storage, and no network calls. It follows the standard pregnancy dating approach used by major medical references: LMP-based dating starts from the first day of the last period, with a typical 40-week pregnancy counted from that point.

It is especially useful when you want a quick estimate that is more informative than a single date. You can compare LMP dating with a scan-based estimate, adjust for cycle length, and see how far along the pregnancy is on any reference date.

How it works

The calculator supports three common ways to estimate a due date:

  1. LMP mode uses the first day of the last menstrual period as the starting point.
  2. Conception mode starts from a known conception date, such as a tracked ovulation date or IVF-related date.
  3. Dating scan mode uses the ultrasound date plus the gestational age measured on that day.

For LMP dating, the standard estimate is 280 days, or 40 weeks, from the first day of the last menstrual period. If the average cycle length is not 28 days, the tool adjusts the estimate by the difference between the cycle length and 28 days.

For conception dating, the calculator adds 266 days, which matches the usual conception-based pregnancy timeline. For scan dating, it back-calculates the likely LMP from the gestational age on the scan date and then derives the estimated due date from that.

The tool also calculates gestational age from the selected reference date, then labels the pregnancy trimester. It shows a due-date window of plus or minus 7 days because real birth timing often varies around the estimate.

Examples

Calculate from last menstrual period

Estimate the due date from the first day of the last period using cycle-length adjustment.

LMP: 2026-01-12, cycle length: 28 days
 
Estimated due date: 2026-10-19, gestational age and trimester shown instantly.

Calculate from dating scan

Back-calculate the estimated due date from an ultrasound date and gestational age.

Scan date: 2026-03-18, scan age: 12 weeks 4 days
 
Estimated due date with calculated LMP, conception estimate, and due-date window.

Calculate from conception date

Use a known conception date, such as IVF or tracked ovulation, to estimate delivery timing.

Conception date: 2026-02-01
 
Estimated due date: 2026-10-24, with gestational age updated from the chosen reference date.

Frequently asked questions

How is the due date calculated from the last menstrual period?

The calculator uses the first day of your last menstrual period as the starting point and adds 280 days, or 40 weeks, to estimate the due date. If your cycle length is not 28 days, it adjusts the result by the cycle difference.

Why does the calculator ask for cycle length?

Cycle length helps refine the estimate when your periods are not on a typical 28-day schedule. A longer cycle can shift ovulation later, while a shorter cycle can shift it earlier.

Can I use this if I know the conception date?

Yes. If you know the conception date, the tool adds 266 days to estimate the due date. This is especially useful for IVF or carefully tracked ovulation dates.

Can I use an ultrasound date instead of a last period?

Yes. In dating scan mode, the calculator uses the scan date and the gestational age measured on that scan to back-calculate the estimated due date. This is helpful when the last period is unknown or uncertain.

Is the due date exact?

No. The due date is an estimate, not a guarantee. Many pregnancies end a little before or after the calculated date, so the calculator also shows a practical date window.

What does gestational age mean?

Gestational age is how far along the pregnancy is, measured from the first day of the last menstrual period. It is shown as weeks and days, which is the most common way pregnancy timing is reported.

What trimester am I in?

The calculator determines the trimester from gestational age. It shows first trimester up to 13 weeks 6 days, second trimester from 14 weeks to 27 weeks 6 days, and third trimester from 28 weeks onward.

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