Gestational Age Calculator
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About Gestational Age
“How far along are you?” has an exact clinical answer, and everything in prenatal care hangs on it: which screening tests are open (and which windows have closed), how ultrasound measurements are judged, when glucose testing happens, and how “preterm,” “term,” and “post-term” get defined. Off-by-a-week matters.
Enter your last period's start date — or switch modes and enter the due date your provider has given you — and you get today's gestational age in the standard weeks-plus-days format, your trimester, and the estimated due date with a countdown. Bookmark it; the answer changes daily.
Don't have a due date yet? Estimate one from LMP, conception, or IVF timing with the Pregnancy Due Date Calculator
How It's Counted
One clock, two ways to read it:
From LMP: GA today = today − first day of last period From due date: GA today = today − (due date − 280 days) Weeks = GA days ÷ 7, remainder shown as days
Worked example: an LMP of March 1, 2026 makes July 10, 2026 exactly 18 weeks, 5 days — second trimester, due December 6, 2026. Entering that due date in due-date mode returns the identical answer, because both modes read the same 280-day clock from opposite ends.
Week-by-Week Milestones
Where the standard checkpoints of US prenatal care fall on the gestational-age clock (individual care plans vary):
| Weeks | Milestone |
|---|---|
| 4 | Missed period; home tests turn reliable |
| 8–10 | Typical first prenatal visit and dating ultrasound |
| 11–14 | First-trimester screening window (NT scan) |
| 14 | Second trimester begins |
| 18–22 | Anatomy scan; movement often first felt |
| 24–28 | Glucose (gestational diabetes) screening |
| 28 | Third trimester begins |
| 37 | Early term |
| 39–40 | Full term — the due date sits at 40w0d |
| 42 | Post-term; induction typically discussed |
Screening windows are the practical reason to know your GA precisely — several tests simply can't be run outside their week ranges.
When the Ultrasound Overrules the Calendar
LMP dating assumes you remember the date and ovulated around day 14 — both fail often enough that ACOG treats first-trimester ultrasound as the most accurate dating method available. If an early scan differs meaningfully from your LMP math, your provider re-dates the pregnancy and that becomes the official clock; enter the resulting due date in this calculator's due-date mode to track against it. Once set, the date doesn't move again just because a later scan measures big or small.
Frequently Asked Questions
How do I calculate my gestational age?
Count the days since the first day of your last menstrual period and divide by 7 — that's weeks and days of gestational age, the convention all prenatal care uses. This calculator does it from either your LMP or a known due date (due date − 280 days recovers the start).
Why does pregnancy counting start before conception?
Because the last period is a known date and conception rarely is. The LMP convention adds roughly two pre-conception weeks to everyone's count — so at “6 weeks pregnant,” the embryo is about 4 weeks old. Every screening window and term definition is built on this clock.
What's the difference between gestational age and fetal age?
Gestational age counts from the LMP; fetal (embryonic) age counts from conception, running about two weeks behind. Clinical care uses gestational age almost exclusively — if a source doesn't specify, it means GA.
What does 18w5d or 18 5/7 mean?
Eighteen completed weeks plus five days of gestational age. Providers count completed weeks — you're “18 weeks” from 18w0d through 18w6d, and the due date lands at exactly 40w0d.
My ultrasound date doesn't match my LMP date — which wins?
The first-trimester ultrasound, per ACOG — early crown-rump measurement outperforms period recall and variable ovulation timing. Once your provider re-dates the pregnancy, use that due date in this calculator's due-date mode.
How many weeks is full term?
ACOG defines early term as 37w0d–38w6d, full term as 39w0d–40w6d, late term 41w0d–41w6d, and post-term from 42 weeks. The due date is the middle of a normal range — only about 1 in 20 babies arrives on it exactly.
Sources & References
- [1](2017). Methods for Estimating the Due Date (Committee Opinion No. 700) — American College of Obstetricians and Gynecologists (ACOG)
Methodology. This calculator uses formulas and health categories recognized by the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC). It is reviewed and maintained by the Vast Calculators editorial team.
Last updated · July 2026
Disclaimer. This tool provides estimates for general informational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider before making decisions about your health.