Pregnancy Week Calculator
Where the count stands — complete weeks and days gestational, the trimester, and the progress toward the 280-day line.
Pregnancy Week Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- LMP convention — no ultrasound inputs
- Trimester lines stated as the common convention
In short: An LMP of Tuesday, 20 May 2026 reads 17 weeks 2 days gestational on the as-of date — the second trimester, 43.214286% of the 280 days gone, 159 left to the due date. The count is complete weeks plus days from the LMP, the way every appointment scheduler reads it; the trimester lines sit at 14w0d and 28w0d on the common convention, stated rather than assumed.
Formula
gestational age = floor(days from LMP ÷ 7) weeks + days; progress = days ÷ 280
Days since the LMP split into complete weeks plus leftover days — 121 days reads 17 weeks 2 days, never “about four months”, because months do not divide the span evenly and appointments run on weeks. The trimester boundaries sit at 14w0d and 28w0d in the common convention; definitions differ by up to a week, so the card states which lines this page draws. The same LMP run forward 280 days is the due date.
Worked Example
- Enter the LMP and the as-of date.
- Read the weeks-and-days count and the trimester.
- Check the progress toward the 280-day line.
Defaults: LMP 20 May 2026 → 17 weeks 2 days on the as-of — second trimester, 43.214286% gone, 159 days left.
Strengths & Limits Of This Model
Where this engine is strong
- Weeks AND days — the scheduler's exact count
- Boundaries stated, not implied
Where it stops
- One LMP, one as-of per run
Practical Use Cases
Appointments
the number the scheduler asks for
Milestones
which trimester line you have crossed
Sharing
weeks and days, the way announcements read
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Pregnancy Week Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why weeks and days instead of months?
Because the calendar's months do not divide the 280 days evenly and appointments do not run on “about four months.” Complete weeks plus days is the count every scheduler, scan report and announcement uses — 17w2d, not 4.
Where are the trimester lines exactly?
On this page: first trimester through 13w6d, second from 14w0d to 27w6d, third from 28w0d. That is the common convention; definitions differ by up to a week depending on which body drew them, so the page states its lines rather than implying a law.
Why does the count start at the LMP?
Convention with a reason: the LMP is the date known to the day, conception rarely is. Gestational age carries a two-week head start over embryonic age — a 17-week count is about 15 weeks from conception — and the same convention prices the due date.
What happens after the 280 days?
The count keeps walking and the page says so: 41w4d reads “past the 280-day mark” with the date that many days behind. Term is a window, not a wall — 38 to 42 weeks is where the overwhelming majority of arrivals land.
Can the count run before the LMP?
No — the LMP sitting ahead of the as-of refuses honestly. Weeks count from a day that happened; a date that has not arrived yet is the due page's job, not this one's.
Is the progress percentage a prediction?
No — it is distance travelled, days over 280, and it stops meaning pace at the line. It is the honest answer to “how far along,” printed to six decimals because the week count beside it is exact.
How does this differ from the due-date page?
Direction again: that page runs the LMP forward to the date and names the cycle adjustment; this one reads the count as of today — weeks, days, trimester, progress. Same convention, opposite questions, and they link to each other.
Do leap days shift the count?
No — the days are real calendar days, 29ths of February included. A count that crosses a leap day simply includes it, which is why the weeks-and-days figure stays exact across years.