pregnancy due date calculator

February 18, 2026

A pregnancy due date calculator estimates when your baby might be born using simple dates you already know, like your last period or conception date.

What a pregnancy due date calculator does

Most online calculators let you plug in one key piece of information and then they estimate your due date and how many weeks pregnant you are.

Common options:

First day of your last menstrual period (LMP).

Conception date (if you know it, for example from ovulation tracking or fertility treatment).

IVF/embryo transfer date on some sites.

They then show:

Estimated due date.
Current week of pregnancy and trimester.
A simple pregnancy timeline or calendar on some tools.

How the due date is calculated

Health sites and calculators use standard medical formulas.

The usual rules:

LMP method: add 280 days (40 weeks) to the first day of your last period, assuming a 28‑day cycle.

Conception method: add 266 days (about 38 weeks) to the conception date.

An easy mental shortcut:

Take the first day of your last period.
Subtract 3 months.
Add 7 days.
This gives the same estimated due date as adding 280 days.

Why it is only an estimate

Your due date is a best guess, not a promise.

Important points:

“40 weeks” is an average; normal pregnancy is usually 37–42 weeks from LMP.

Ovulation does not always happen on day 14, especially with irregular cycles, so the real conception date may shift the true due date.

Only a small percentage of babies are actually born on the calculated due date.

Because of this, your care provider will:

Check the date from your LMP.
Compare it with early ultrasound measurements (which can adjust the due date if needed).

Simple example

Imagine your last period started on 1 March:

Add 280 days → estimated due date early December.

Or: March 1 → subtract 3 months (December 1), then add 7 days → December 8 as the estimated due date.

If you instead know conception was around 15 March, adding 266 days points you to late November.

Quick HTML table summary

Below is an HTML table (as requested) summarizing the main calculation methods:

html

<table>
  <thead>
    <tr>
      <th>Method</th>
      <th>What you enter</th>
      <th>How it calculates</th>
      <th>Notes</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>LMP (last menstrual period)</td>
      <td>First day of your last period</td>
      <td>Add 280 days (40 weeks) to that date</td>
      <td>Most common method; assumes 28-day cycle and ovulation around day 14.[web:1][web:3][web:5][web:7][web:9]</td>
    </tr>
    <tr>
      <td>Conception date</td>
      <td>Date you likely conceived</td>
      <td>Add 266 days (about 38 weeks)</td>
      <td>Useful if you tracked ovulation closely or had fertility treatment.[web:1][web:3][web:5]</td>
    </tr>
    <tr>
      <td>IVF/transfer date</td>
      <td>Embryo transfer date and embryo age</td>
      <td>Calculator adjusts based on embryo’s developmental age</td>
      <td>Offered on some medical or fertility-oriented calculators.[web:1]</td>
    </tr>
    <tr>
      <td>Official clinical dating</td>
      <td>LMP plus early ultrasound</td>
      <td>Provider may revise due date using ultrasound measurements</td>
      <td>Considered the most reliable for your medical chart.[web:1][web:5]</td>
    </tr>
  </tbody>
</table>

Where to use a pregnancy due date calculator

You can try trusted calculators from:

Major pregnancy organizations and charities.

Hospital or clinic websites.

Large baby and parenting brands that host simple calculators.

These tools are great for planning, but always confirm dates and any concerns with your doctor or midwife.

TL;DR: A pregnancy due date calculator uses your last period or conception date to estimate when your baby may arrive, typically by adding 280 days from LMP or 266 days from conception. Always treat it as an estimate and check with your healthcare provider for personalized advice.

Information gathered from public forums or data available on the internet and portrayed here.