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.