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Health & Fitness

Pregnancy Week Calculator

Track your pregnancy week and due date

Reviewed: AnyCalc Editorial|2026-07-19|Editorial policy|About

What this calculator does

A pregnancy week calculator determines how far along you are in your pregnancy and estimates your due date based on the first day of your last menstrual period (LMP). The standard method, used by obstetricians worldwide, calculates your due date as 280 days (40 weeks) from your LMP using Naegele's rule: add 7 days to your LMP date, then add 9 months (or subtract 3 months). At each week, different developmental milestones occur — for instance, the heart begins beating around week 6, organs form during the first trimester, and the baby reaches viability around weeks 24–26. This tool displays your current gestational week, trimester, estimated due date, and key developmental highlights for that stage. It is useful for expectant parents tracking milestones, preparing for prenatal appointments, and understanding what to expect at each stage. First-trimester ultrasound dating is more accurate than LMP-based calculations, so if your ultrasound date differs significantly, consult your healthcare provider. The calculator also helps you count down weeks remaining and plan maternity leave or nursery preparation timelines.

How to use

  1. 01
    Enter your date
    Input the first day of your last menstrual period or your due date.
  2. 02
    Click Calculate
    Press Calculate to determine your current pregnancy week.
  3. 03
    View milestones
    See your trimester, estimated due date, and weekly development details.

Pregnancy Week Calculator FAQ

How is pregnancy due date calculated?▾
The most common method is Naegele's rule: take the first day of your last menstrual period, add 7 days, then add 9 months (or equivalently, add 280 days). This assumes a 28-day cycle with ovulation on day 14. Your doctor may adjust the date based on a first-trimester ultrasound measurement.
What trimester am I in?▾
The first trimester spans weeks 1–12, the second trimester covers weeks 13–26, and the third trimester runs from week 27 until delivery around week 40. Each trimester has distinct developmental milestones and common symptoms, from morning sickness in the first to Braxton Hicks contractions in the third.
How accurate are pregnancy due dates?▾
Only about 5% of babies are born on their exact due date. Most deliveries occur within two weeks before or after the estimated date. First-trimester ultrasound dating is accurate to within about 5–7 days, while LMP-based calculations can be off by more if your cycles are irregular.
Does pregnancy really last 9 months?▾
Pregnancy is officially counted as 40 weeks from the last menstrual period, which is closer to 9 months and one week. However, the first two weeks of that count are before conception actually occurs, so the actual time from fertilization to birth is about 38 weeks.

Interpret carefully

Outputs are decision-support estimates, not a substitute for professional judgment.

  • Treating Pregnancy Week Calculator output as a final professional diagnosis, quote, or legal determination
  • Entering approximate values when the result is sensitive to units or precision
  • Applying the FAQ answer (“How is pregnancy due date calculated?”) to a special case without checking assumptions

Next steps

  1. Double-check inputs against your source documents (labs, statements, specs, or notes).
  2. Confirm the method below matches your use case before you act on the number.
  3. For medical, financial, or production decisions, verify with a qualified professional.

Disclaimer: Results are informational only and do not constitute medical, financial, legal, or professional advice. Report issues: sublimernj@gmail.com

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