Formula
Pregnancy week formula
Gestational age = days since first day of last menstrual period / 7A 40-week estimate is commonly counted from the first day of the last menstrual period.
Pregnancy Week Calculator
Use this pregnancy week calculator to estimate gestational age from the first day of the last menstrual period.
Planning estimate
Loading calculator with your local date and time…
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.
How to use
Steps
- Enter the first day of the last menstrual period.
- Enter average cycle length if it differs from 28 days.
- Choose the date you want to calculate as of.
- 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
- 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.
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
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.
Benchmarks
How to read the result
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
Formula, inputs, limitations and review record
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.
Review record
Method and sources checked on July 12, 2026. The documented method, source timing, validity conditions, and reliance limits were checked on this date.
Method provenanceEstimate · toolkitshelf.pregnancy-week-LMP.v1
- Maintained by
- Toolkit Shelf
- Calculation class
- Estimate
- Method version
- toolkitshelf.pregnancy-week-LMP.v1
- Primary source
- ACOG Committee Opinion No. 700
- Source date
- May 2017; reaffirmed 2025
- Source checked
- July 12, 2026
- Intended audience
- People making a rough gestational-age estimate from a known last menstrual period and chosen as-of date.
- Valid when
- The estimate assumes the last menstrual period and as-of date are correct. Assisted reproduction, irregular cycles, and ultrasound findings require clinical interpretation.
- Reliance boundary
- This is an LMP-based planning estimate, not clinical gestational dating, prenatal care, or a replacement for ultrasound and clinician review.
- Data freshness
- ACOG Committee Opinion No. 700 and its reaffirmed status were checked on July 12, 2026.
Cite this page
Toolkit Shelf. Pregnancy Week Calculator. 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.
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