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Pregnancy Calculator

Enter your values below to get the result first, then scroll for the full explanation and guidance.

Step 1 • Add values

Use the calculator

Enter your values below to generate an instant result. You can update the inputs at any time to compare different scenarios.

Example: 15 January 2026 LMP, 28-day cycle and 21 April 2026 gives an estimated pregnancy age of 13 weeks 5 days.

Results refresh instantly as values change.

Estimated pregnancy age

13 weeks 5 daysSecond trimester

Estimated pregnancy age: 13 weeks 5 days (Second trimester)

This estimates gestational age from the first day of the last menstrual period, adjusted for the entered regular cycle length. It is not a clinical dating result.

Pregnancy timeline

This estimates gestational age from the first day of the last menstrual period, adjusted for the entered regular cycle length. It is not a clinical dating result.

Result snapshot

A quick visual read of the values behind this result.

LMP date15 January 2026
Cycle length28 days
Estimated due date22 October 2026

Recommended next checks

  • →Check the due-date view if you want the estimated delivery date from the same LMP.
  • →Use scan-based dating if your maternity team has advised a different gestational age.
LMP date
15 January 2026
Cycle length
28 days
Estimated due date
22 October 2026

Try different values to compare results.

Enter the first day of your last period, your usual regular cycle length and the date you want to check. The result is an LMP-based gestational-age estimate, not a diagnosis or a substitute for antenatal dating.

Fast to use

Built for comparison

Clear result output

Table of Contents

5

About Pregnancy Calculator

Pregnancy Calculator helps you work through the main numbers for this topic quickly with a simple input flow and an instant result.

Use the calculator result as a practical starting point, then review the explanation and assumptions on the page if you want more context.

How this pregnancy estimate works

A 28-day cycle uses 280 days from the first day of the last menstrual period. A longer or shorter regular cycle shifts the estimated due date by the difference from 28 days. For a 28-day cycle, 15 January to 21 April 2026 is 13 weeks 5 days.

Pregnancy commonly lasts 37 to 42 weeks from the LMP. NHS guidance says the 12-week scan can estimate gestation more accurately. If your LMP is uncertain, cycles are irregular or your maternity team has supplied a scan date, use their clinical date instead.

Formula explained

Calculation flow

This calculator is structured for fast UK-focused estimates with clear inputs, repeatable logic, and instant results.

Formula

Input values -> calculation engine -> instant result

How the result is built

1Enter the values requested in the form.
2The calculator applies the configured formula logic.
3The result updates instantly with a breakdown.
4Use the output to compare scenarios quickly.

Example

Example: 15 January 2026 LMP, 28-day cycle and 21 April 2026 gives an estimated pregnancy age of 13 weeks 5 days.

Assumptions

  • Gestational dating starts from the first day of the last menstrual period; the 280-day due-date estimate is shifted by cycle length minus 28 days.
  • Supports regular cycles from 21 to 35 days and dates from 0 to 42 completed weeks after the adjusted LMP only.
  • This is not fetal age or clinical confirmation. Irregular cycles, uncertain LMP, assisted conception and scan-based dating require clinical information not modelled here.

Source basis

  • UK-focused calculator flow
  • Structured input validation
  • Instant result breakdowns

Trust and notes

Assumptions and important notes

This calculator is designed to give a fast estimate using the method shown on the page. Results are most useful when your inputs are accurate and the tool matches your situation.

Use the result as guidance rather than a final diagnosis or professional decision. If the result could affect health, legal, financial, or compliance decisions, verify it with a qualified source where appropriate.

  • Gestational dating starts from the first day of the last menstrual period; the 280-day due-date estimate is shifted by cycle length minus 28 days.
  • Supports regular cycles from 21 to 35 days and dates from 0 to 42 completed weeks after the adjusted LMP only.
  • This is not fetal age or clinical confirmation. Irregular cycles, uncertain LMP, assisted conception and scan-based dating require clinical information not modelled here.

Method

UK calculator guidance

Last reviewed

April 17, 2026