When you walk out of your first-trimester ultrasound, the sonographer hands you a picture and a number: a crown-rump length (CRL) in millimeters, and a gestational age like "12 weeks 6 days." That number is usually more accurate than the due date you worked out from your last period, especially if your cycles are irregular, you were breastfeeding, or you simply cannot remember the first day of your last period. This ultrasound due date calculator takes the scan date and either the CRL or the gestational age on your report and recovers your estimated due date (EDD) directly.
The engine works backwards: it subtracts the gestational age at the scan from the scan date to recover your estimated last menstrual period (LMP), then adds the standard 280 days (40 weeks) of a full-term pregnancy. This is the same method obstetricians use when an early scan disagrees with the LMP-based date, and it is why a first-trimester ultrasound is considered the gold standard for pregnancy dating.
How to Use
- Enter the Ultrasound Scan Date — the exact calendar date printed at the top of your ultrasound report.
- Enter the Crown-Rump Length (CRL) in millimeters. This is the length of the embryo from the top of the head (crown) to the bottom of the torso (rump). It is almost always labeled "CRL" on the report.
- If the report already states a gestational age, also enter Gestational Weeks at Scan and Extra Gestational Days. The calculator uses these as a cross-check, and uses them directly when you set CRL to 0.
- Press calculate. The result card shows your estimated due date, the recovered LMP, how many weeks and days pregnant you were on the scan date, and how many days from the scan until the due date.
- If your scan was after about 14 weeks, the CRL number alone becomes less reliable; enter the gestational age the sonographer reported instead and leave CRL at 0.
A note on accuracy: a first-trimester ultrasound (before 13 weeks 6 days) predicts the due date to within about ±5 to 7 days. By the second trimester that window widens to ±10 to 14 days, and by the third trimester ultrasound dating is not used to change the EDD at all.
How the CRL Becomes a Due Date
The Robinson and Fleming (1975) curve, still used in most obstetric units, relates CRL in millimeters to gestational age in days:
GA (days) = 8.052 × √(CRL × 1.037) + 23.73
For example, a CRL of 55 mm gives 8.052 × √(57.04) + 23.73 ≈ 8.052 × 7.55 + 23.73 ≈ 85 days, or about 12 weeks 1 day. Once the gestational age at the scan is known, the rest is simple calendar arithmetic:
- Recovered LMP = scan date − gestational age at scan
- Estimated due date = recovered LMP + 280 days
If your scan was on 1 April 2026 and the CRL was 55 mm (≈ 12 weeks 1 day, 85 days pregnant), the recovered LMP is 6 January 2026, and adding 280 days gives an EDD of 13 October 2026. That is the number your obstetrician will write on your antenatal chart.
Why Ultrasound Dating Beats LMP
Naegele's rule (LMP + 280 days) assumes a textbook 28-day cycle with ovulation on day 14. In real life, roughly 20% to 30% of cycles are irregular, ovulation can happen anywhere from day 10 to day 20, and many women simply cannot recall the first day of their last period. An early ultrasound measures the embryo itself, which grows at a remarkably consistent rate in the first trimester, so it does not care about your cycle history.
ACOG (the American College of Obstetricians and Gynecologists) recommends that if an early ultrasound disagrees with the LMP-based due date by more than a set threshold (typically more than 5 to 7 days in the first trimester), the ultrasound date should replace the LMP date on the chart. This calculator gives you that ultrasound-based EDD in seconds.
Examples
Example 1 — The textbook early scan. A patient has her dating scan on 1 April 2026. The report shows a CRL of 55 mm, which the Robinson curve reads as a gestational age of about 12 weeks 1 day. The calculator recovers an LMP of 6 January 2026 and an estimated due date of 13 October 2026, with 195 days remaining from the scan.
Example 2 — Irregular cycles, no known LMP. A woman with 35- to 45-day cycles cannot remember her last period. Her 8-week scan shows a CRL of 17 mm. Entering the scan date and CRL (with weeks and days left at 0) gives a recovered LMP and an EDD that the LMP method could never have produced — because she would have guessed her ovulation date wrong by weeks.
Example 3 — Second-trimester anatomy scan. At 20 weeks, the sonographer reports a gestational age of 19 weeks 5 days on 10 June 2026. Set CRL to 0, enter 19 weeks and 5 days, and the calculator recovers the LMP and EDD directly. The CRL curve is not reliable past about 14 weeks, which is why the calculator falls back to the reported gestational age in that range.
Example 4 — Large CRL sanity check. If you type a CRL above 95 mm (well outside the validated first-trimester range), the calculator ignores the CRL extrapolation and uses the weeks/days your sonographer reported instead, so you never get a nonsense due date from a third-trimester measurement.
FAQ
How accurate is an ultrasound due date compared to LMP?
A first-trimester ultrasound (before 13 weeks 6 days) is accurate to within about ±5 to 7 days, which is more precise than Naegele's rule for anyone with irregular cycles, ovulation on a day other than cycle day 14, or an uncertain memory of their last period. After 14 weeks the accuracy drops to ±10 to 14 days, and after 28 weeks ultrasound is not used to change the due date.
What is CRL and why does it matter?
CRL stands for crown-rump length, the distance from the top of the embryo's head to the bottom of its torso, measured on a first-trimester ultrasound. Because early embryos grow at a nearly constant rate, CRL is the single best predictor of gestational age in the first trimester. It is usually reported in millimeters on your ultrasound report.
What if my ultrasound report already says how many weeks pregnant I am?
Enter that number directly. Set CRL to 0, type the weeks and extra days into the two "Gestational Age" fields, and enter the scan date. The calculator subtracts that gestational age from the scan date to recover your LMP, then adds 280 days. This is the most accurate path when the sonographer has already done the CRL measurement for you.
Why does my ultrasound due date differ from my LMP due date?
The LMP method assumes you ovulated on day 14 of a 28-day cycle. If you ovulated earlier or later, or if your cycles are longer or shorter than 28 days, the LMP date will drift. An early ultrasound measures the embryo directly and does not make that assumption. ACOG recommends adopting the ultrasound date when the two differ by more than 5 to 7 days in the first trimester.
Can I use this calculator for a second- or third-trimester scan?
Yes, but enter the gestational age the sonographer reported rather than a CRL number. The Robinson CRL curve is only validated between roughly 7 and 14 weeks (CRL 15–95 mm). Past that, fetal growth varies too much for a single length measurement to date the pregnancy reliably, and your clinician will use the reported gestational age instead.
What if I don't know the exact scan date?
Use the date printed on the ultrasound report itself — it is almost always at the top. If you cannot find it, the EDD will shift by exactly one day for each day your scan date is off, so a one-day error is harmless. A week-long error in the scan date will shift the due date by a week, so try to get the date right.
Does the ultrasound due date change as the pregnancy progresses?
No. The EDD from a first-trimester scan is locked in at that scan and does not change on later ultrasounds. Later scans may show the baby measuring ahead or behind, but that reflects fetal growth, not a new due date. Your obstetrician will keep the original first-trimester EDD on file.
Disclaimer
This ultrasound due date calculator is provided for informational purposes only and is not medical advice. It is a planning and educational tool, and the estimated due date it produces is an approximation based on the scan date, CRL, and gestational age you enter. Due dates are estimates; only about 5% of babies are born on their exact due date, and normal delivery can occur anywhere from 37 to 42 weeks. Always confirm your official due date, dating decisions, and any pregnancy concerns with your obstetrician or midwife, who has access to your full clinical history and the actual ultrasound images.