● Pregnancy & wellness

Gestational age in weeks and days.

Enter your last menstrual period date or an ultrasound measurement to get gestational age, trimester, estimated due date and a visual milestone timeline.

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Gestational Age Calculator

Weeks, days, trimester, due date and milestone timeline

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Your data

Dating method

First day of last menstrual period (LMP)

Today's date — auto-filled, adjust if needed

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Your result

— wk

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Gestational age—

Trimester—

Estimated due date EDD — Naegele's rule—

Days until due date—

Conception (approx.)—

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Pregnancy timeline & milestones

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How it's calculated

LMP dating and Naegele's rule — the standard method

Gestational age is measured in weeks and days from the first day of the last menstrual period (LMP), not from conception. This calculator uses Naegele's rule — LMP + 280 days = estimated due date (EDD) — which is endorsed by ACOG. When an ultrasound date is provided, the calculator back-computes the equivalent LMP and applies the same formula. For example, an LMP of January 1 gives an EDD of October 8 and a gestational age of 40 weeks at delivery.

Gestational age starts from the LMP because clinicians historically knew this date precisely, whereas ovulation (and conception) occurs ~14 days later. A full-term pregnancy is 40 weeks from LMP (280 days). First-trimester ultrasound using crown-rump length (CRL) is the gold standard for dating when LMP is uncertain.

EDD (Naegele's rule) = LMP + 280 days
Gestational age = Today − LMP (in weeks + days)
Ultrasound → back-compute LMP = US date − (GA at US in days)

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Determine reference LMP (from input or back-computed from ultrasound)
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2
Calculate days elapsed since LMP as of today
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Convert to weeks + remaining days
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Add 280 days to LMP to get EDD
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Understand the terms

Gestational age: Age of a pregnancy counted in weeks and days from the first day of the last menstrual period (LMP). Standard reference for all prenatal care.
Trimester: One of the three roughly 13-week periods of pregnancy: 1st (weeks 1–13), 2nd (weeks 14–27), 3rd (weeks 28–40+).
Viability: The gestational age at which a fetus has a reasonable chance of survival outside the womb, conventionally set at 24 weeks (with intensive neonatal care).
Full term: ACOG defines full term as 39 weeks 0 days through 40 weeks 6 days. Births from 37 to 38w6d are "early term"; from 41w, "late term".
Preterm: Birth before 37 completed weeks of gestation. The WHO classifies preterm as extremely (<28w), very (28–32w) and moderate-to-late (32–37w).

See the full glossary →

Disclaimer: estimation tool for informational and planning purposes, based on standard obstetric dating (Naegele's rule, 2026 ACOG guidelines). The actual gestational age and due date must be confirmed by your obstetrician or midwife via clinical assessment and ultrasound. This calculator does not substitute for professional prenatal care.

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Frequently asked questions — Gestational Age

What is gestational age?

Gestational age is the age of a pregnancy measured in weeks and days, counted from the first day of the last menstrual period (LMP). It differs from fetal age (counted from conception, approximately 2 weeks later). All prenatal care milestones, test timing, and due-date calculations use gestational age as their reference.

How is the estimated due date (EDD) calculated?

Using Naegele's rule, the EDD = LMP + 280 days (40 weeks). This assumes a 28-day menstrual cycle with ovulation on day 14. When cycles are irregular or the LMP is uncertain, first-trimester ultrasound (crown-rump length before 14 weeks) provides a more accurate EDD within ±5–7 days.

What are the three trimesters of pregnancy?

The first trimester covers weeks 1–13 (major organ formation). The second trimester covers weeks 14–27 (growth, movement felt, anatomy scan at ~20w). The third trimester covers weeks 28 through delivery (~40w), during which the baby gains most of its weight and lungs mature.

When is a baby considered full term?

ACOG defines full term as 39 weeks 0 days through 40 weeks 6 days. Early term is 37w0d–38w6d; late term is 41w0d–41w6d; post-term is 42 weeks or beyond. Babies born before 37 weeks are classified as preterm and may require additional neonatal support.

How accurate is ultrasound dating compared to LMP?

First-trimester ultrasound (CRL before 14 weeks) is the most accurate dating method, with a margin of ±5–7 days, outperforming LMP-based dating (which assumes regular 28-day cycles). ACOG and ISUOG recommend ultrasound-based EDD when the difference between LMP and CRL dating exceeds 5 days (1st trimester) or 7 days (2nd trimester).

📚 Learn more — official sources

📋 Guidelines ACOG Committee Opinion 700 — Methods for Estimating the Due Date American College of Obstetricians and Gynecologists official guidance on LMP, ultrasound dating and when to revise the EDD. See official source → 📋 Guidelines ISUOG — First Trimester Ultrasound Practice Guidelines International Society of Ultrasound in Obstetrics and Gynecology guidelines on crown-rump length measurement and gestational age assignment. See official source → 🏛️ Organization WHO — Preterm Birth Fact Sheet World Health Organization definitions, statistics and interventions for preterm birth (<37 weeks gestational age). See official source → 🔬 Study StatPearls — Gestational Age Assessment (NCBI Bookshelf) Peer-reviewed reference on methods for assessing gestational age from LMP, ultrasound and physical examination in clinical practice. See on PubMed →

Gestational Age Calculator

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About the Gestational Age Calculator

Gestational age is the universal clock of pregnancy — every prenatal appointment, scan, test and delivery plan references it. This calculator implements the two most common dating methods endorsed by ACOG: LMP-based dating (Naegele's rule, +280 days) for routine use, and ultrasound back-dating for cases where the LMP is uncertain or cycles are irregular.

The milestone timeline shows which events have already occurred (cardiac activity ~6w, NT scan ~12w, anatomy scan ~20w, viability ~24w, full term ~39w) based on the calculated gestational age. All calculations happen locally in your browser — no data is sent anywhere.