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Health Calculators

Pregnancy Week Calculator

Use this pregnancy week calculator to estimate gestational age from the first day of the last menstrual period.

Last reviewed June 6, 2026Source note includedPlanning estimateNo expert review claimed

Live calculator

Pregnancy week

Estimated gestational age9w 6d

Approximate first trimester based on last menstrual period.

Estimated due dateFeb 5, 2027

211 days remaining in a 40-week estimate.

Care note

Pregnancy dating can change after clinical review or ultrasound. Use this for rough planning and confirm dates with a qualified clinician.

Use this as a planning estimate, not medical advice. Medical conditions, medication, pregnancy, eating disorder history, or disease-specific nutrition needs should be handled with a qualified professional.

Quick answer

Pregnancy Week Calculator: what it calculates

Pregnancy Week Calculator calculates estimated gestational age from last menstrual period, cycle length, and as-of date. The visible formula is Gestational age = days since first day of last menstrual period / 7.

ResultEstimated gestational age
InputsLast menstrual period, Cycle length, As-of date
FormulaPregnancy week formula

Formula

Pregnancy week formula

Gestational age = days since first day of last menstrual period / 7

A 40-week estimate is commonly counted from the first day of the last menstrual period.

How to use

Steps

  1. Enter the first day of the last menstrual period.
  2. Enter average cycle length if it differs from 28 days.
  3. Choose the date you want to calculate as of.
  4. Review estimated weeks, days, trimester, and due date.

Example

Sample calculation

Last periodMay 1, 2026
As ofMay 26, 2026
Pregnancy week3w 4d

Calculator use

Best for

  • Use this pregnancy week calculator to estimate gestational age from the first day of the last menstrual period.
  • Estimating due date, pregnancy week, trimester, or conception timing for planning conversations.
  • Checking date assumptions from last menstrual period, due date, or cycle length with the method visible.
  • Preparing non-diagnostic timing context before confirming dates with a qualified clinician.

Before relying on it

Check first

  • Treating pregnancy date estimates as diagnosis, medical advice, or a substitute for clinician dating.
  • Ignoring cycle variability, ultrasound timing, fertility treatment dates, irregular periods, or clinician guidance.
  • Using late or uncertain dates as if they were exact conception or due-date proof.

Details

What to know before using the result

These notes make the assumptions explicit, especially where the same search query can mean slightly different things.

Dating methodLMP-based estimate

Gestational age is commonly counted from the first day of the last menstrual period, not from the estimated conception date.

Clinical datingMay change

A clinician may revise dates using ultrasound, cycle history, assisted reproduction dates, or other clinical information.

Cycle adjustmentRough personalization

Cycle length can shift estimated ovulation and due date timing, but irregular cycles can make estimates less reliable.

Source notes

References used for this calculator

ACOGStates that estimated due date is calculated from the first day of the last menstrual period and average pregnancy length is 280 days or 40 weeks.ACOGProvides clinical guidance on methods for estimating the due date and documenting gestational age.

Benchmarks

How to read the result

The calculator is a decision aid, not a fixed rule. Use the output to compare scenarios and document your assumptions. Benchmark ranges are broad planning heuristics unless this page names a specific source for the range.

0 - 13 weeks: First trimester.

A broad trimester reference used by the calculator.

14 - 27 weeks: Second trimester.

Dating should still be confirmed clinically.

28+ weeks: Third trimester.

Late-pregnancy dating by ultrasound can be less reliable than early dating.

Calculator accuracy

Methodology and assumptions

The formula, inputs, example, and limitations are shown so the result is checkable, not just a number in a box.

Formula

Gestational age = days since first day of last menstrual period / 7

Inputs used

Last menstrual period, Cycle length, As-of date

Limitations

Health results are screening or planning estimates. Age, body composition, medication, medical history, pregnancy, and professional guidance can change the right interpretation.

Last reviewed

June 6, 2026

Cite this page

Toolkit Shelf. Pregnancy Week Calculator. Last reviewed June 6, 2026. https://toolkitshelf.com/tools/pregnancy-week-calculator

FAQ

Common questions

How do I calculate how many weeks pregnant I am?

Count the days from the first day of the last menstrual period to the as-of date, then divide by seven.

Is pregnancy counted from conception?

Gestational age is commonly counted from the first day of the last menstrual period, which is usually about two weeks before conception.

Can my due date change?

Yes. A clinician may update the estimated due date after reviewing ultrasound and clinical information.

When should I ask a clinician?

Ask a clinician or qualified nutrition professional when pregnancy, medication, medical history, eating disorder risk, symptoms, or disease-specific guidance matters.

Why can health formulas be misleading?

Health formulas simplify age, sex, body composition, training status, measurement method, pregnancy, and medical context into a planning estimate.

Is this medical advice?

No. Health calculators are screening or planning tools and do not diagnose, treat, or replace professional medical or nutrition guidance.