Pregnancy Week to Due Date Calculator
Already know how many weeks along you are? Turn your gestational age from a scan or appointment into an estimated due date, the implied last period and conception date, how far along you are today, your trimester, term dates, a milestone timeline and a week-by-week calendar — with an optional ACOG check against your last period.
How it works
gestational age = weeks × 7 + extra days · dating start (estimated LMP) = as-of date − gestational age · due date = dating start + 280 days (= as-of date + 280 − gestational age) · conception ≈ dating start + 14 days · progress on any date = that date − dating start · ACOG comparison: keep the last-period dates unless they differ from the scan by more than 5 days (before 9 weeks), 7 (9–15w6d), 10 (16–21w6d), 14 (22–27w6d) or 21 days (28 weeks on)
A full-term pregnancy is dated as 280 days (40 weeks) from the first day of the last menstrual period, so if you already know your gestational age on a given date — from a scan report (“12w3d”) or an appointment — the calculator works forwards to the due date and backwards to the implied last period and conception date. It then shows how far along you are today (or on any date you pick), your week, trimester and month, the share of the 40 weeks behind you and the days left. Because a due date is really a window, it also gives the full-term dates (39 weeks 0 days to 40 weeks 6 days) and the wider 37–42-week term window, a timeline of common scans and tests with their dates, and a week-by-week calendar. If the weeks came from an ultrasound and you also know your last period, switch on the comparison: following the American College of Obstetricians and Gynecologists (ACOG), the due date from your period is kept unless the two disagree by more than 5 to 21 days depending on how early the scan was, in which case the scan dates are used.
Worked example
If you are 12 weeks 3 days along on October 19, 2026, that is 87 gestational days. The implied last period began 87 days earlier, on July 24, 2026, and the due date is 280 − 87 = 193 days later: Friday, April 30, 2027. Conception was around August 7, 2026, you are in week 13 of the first trimester, full term runs from April 23 to May 6, 2027, and the anatomy scan window (18–22 weeks) is November 27 to December 25, 2026. If your last period actually started on July 20, 2026, it puts you at 13 weeks 0 days on October 19 — 4 days more than the scan. ACOG’s limit at 12 weeks is 7 days, so the period-based due date of Monday, April 26, 2027 is kept.
Sources: ACOG Committee Opinion 700 — Methods for Estimating the Due Date · ACOG Committee Opinion 579 — Definition of Term Pregnancy
Frequently asked questions
Why does this ask for weeks rather than my last period?
Because scans and clinicians often report progress as “X weeks Y days”. Entering that directly avoids re-deriving it from a date. If you also know your last period, switch on the comparison under More options to see both due dates and which one ACOG’s rule keeps.
How accurate is a due date from gestational age?
It is an estimate. Only about 4% of babies arrive on the exact due date; most births fall within roughly two weeks either side. An ultrasound before 14 weeks dates a pregnancy to within about 5–7 days, which is why it usually takes priority over the last period when the two disagree by more than that.
When should the scan date replace the last-period date?
ACOG recommends changing the due date when the scan and the last period differ by more than 5 days before 9 weeks, 7 days from 9 to 15 weeks 6 days, 10 days from 16 to 21 weeks 6 days, 14 days from 22 to 27 weeks 6 days and 21 days from 28 weeks. The calculator applies exactly those limits.
Which trimester am I in?
This tool uses the ACOG convention: the first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks to birth. Some sources split at 13 and 27 weeks instead, so a week either side of the boundary can be labelled differently.
What is the difference between 12 weeks and week 13?
They describe the same day. “12 weeks 3 days” counts completed weeks; you are then in your 13th week. The calculator shows both.
Related calculators
Educational estimate only — not medical advice. Confirm dating and due date with your healthcare provider.