MasterTool

Gestational Age Calculator

Work out how far along a pregnancy is — weeks and days — from the last menstrual period to any date.

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What this tool does

Work out how far along a pregnancy is — weeks and days — from the last menstrual period to any date. Calendar arithmetic from LMP assuming a 28-day cycle. Dating by early ultrasound can differ and takes clinical precedence.

How to use the Gestational Age Calculator

  1. Enter or select first day of last menstrual period (yyyy-mm-dd).
  2. Enter or select calculate as of date (yyyy-mm-dd).
  3. Read the calculated result; change any measurement to compare alternatives.

Formula

gestational age = days between LMP and the as-of date, expressed as whole weeks + remaining days; due date = LMP + 280 days; trimester: 1st weeks 0–13, 2nd 14–27, 3rd 28+
lmp
First day of last menstrual period (YYYY-MM-DD)
asof
Calculate as of date (YYYY-MM-DD)

Calendar arithmetic from LMP assuming a 28-day cycle. Dating by early ultrasound can differ and takes clinical precedence.

Worked example

For gestational age calculator, the following measurements illustrate the exact method: First day of last menstrual period (YYYY-MM-DD): 2026-08-20; Calculate as of date (YYYY-MM-DD): 2026-10-07.

Inputs

  • First day of last menstrual period (YYYY-MM-DD)2026-08-20
  • Calculate as of date (YYYY-MM-DD)2026-10-07

Result

  • Gestational age6 weeks 6 days
  • Total days48
  • TrimesterFirst
  • Estimated due date (LMP + 280 days)2027-05-27

Results explained

Gestational age
Gestational age from the formula above. Calendar arithmetic from LMP assuming a 28-day cycle. Dating by early ultrasound can differ and takes clinical precedence.
Total days
Total days from the formula above. Calendar arithmetic from LMP assuming a 28-day cycle. Dating by early ultrasound can differ and takes clinical precedence.
Trimester
Trimester from the formula above. Calendar arithmetic from LMP assuming a 28-day cycle. Dating by early ultrasound can differ and takes clinical precedence.
Estimated due date (LMP + 280 days)
Estimated due date (LMP + 280 days) from the formula above. Calendar arithmetic from LMP assuming a 28-day cycle. Dating by early ultrasound can differ and takes clinical precedence.

Frequently asked questions

Count the days from the first day of the last menstrual period to today (or your chosen date), then express them as weeks plus remaining days — e.g. 48 days is 6 weeks 6 days.

By convention, gestational age starts at the LMP because that date is usually known, while conception day is not. It makes a '40-week' pregnancy include about 2 pre-conception weeks.

Commonly: first trimester weeks 0–13, second 14–27, third 28 until birth.

Early ultrasound dating (especially before 14 weeks) is generally considered more accurate than LMP recall and cycle assumptions, and clinicians will use it. This calculator performs LMP arithmetic only.

LMP-based age overestimates slightly with long cycles because ovulation comes later. Note the difference for your clinician; the due date calculator on this site adjusts by cycle length.