An ultrasound due date calculator derives your estimated due date from a scan: you enter the scan date and the gestational age the sonographer measured (or a CRL measurement for early scans), and the tool adds the remaining days to reach 40 weeks. Because it measures the baby directly rather than relying on remembered dates, ultrasound dating is the most accurate method available.
This is especially handy when your LMP is uncertain, your cycles are irregular, or your provider has just handed you a scan report with a gestational age in weeks and days. Rather than doing the calendar arithmetic yourself, the calculator works backward from the scan to your due date in one step.
How to Use the Ultrasound Due Date Calculator
Find the scan date on your ultrasound report — the day the scan was performed, not the day you received the report.
Enter the gestational age your sonographer recorded (for example, 10 weeks 2 days). If your report shows only a CRL in millimeters, use the CRL option instead.
Get your estimated due date — the tool computes scan date + (280 days − gestational age in days) instantly.
Compare with your LMP-based date using the LMP due date calculator — a small difference is normal; a gap of more than a week is worth discussing with your provider.
Use your earliest scan for dating. Later scans monitor growth rather than age, so the first-trimester measurement is the one that sets the official date.
How the Calculation Works
EDD = Scan Date + (280 days − Gestational Age in days at scan)
Pregnancy is measured as 280 days of gestational age from the last period. If the scan shows the baby at a gestational age of G days, then 280 − G days of the pregnancy remain. Adding those remaining days to the scan date lands on the estimated due date.
For early scans, the calculator first converts CRL (crown-rump length) into gestational age using standard reference values, then applies the same formula. Approximate conversions used here:
CRL Reference Chart
CRL (mm)
Gestational Age
Weeks + Days
10 mm
50 days
7w 1d
20 mm
56 days
8w 0d
30 mm
69 days
9w 6d
45 mm
80 days
11w 3d
60 mm
87 days
12w 3d
70 mm
91 days
13w 0d
84 mm
97 days
13w 6d
These conversions are approximate — clinical charts vary slightly, and your sonographer’s own measurement is the authoritative one for your pregnancy.
What Happens at a Dating Scan
A dating scan usually takes place between 8 and 12 weeks of pregnancy, often at your first prenatal visit. The appointment is short — typically 15 to 30 minutes — and its goal is straightforward: measure the pregnancy, confirm the heartbeat, and set the official estimated due date.
What the sonographer measures
The key measurement is the crown-rump length (CRL), taken with the embryo in a straight, neutral position. The sonographer may also note the gestational sac, the yolk sac, and the fetal heart rate, and will count the number of embryos. Each of these details feeds into the gestational age estimate on your report, usually expressed as weeks and days.
Transabdominal vs transvaginal
Very early scans (around 6 to 8 weeks) are often done transvaginally, because the probe sits closer to the uterus and produces a clearer image when the embryo is tiny. Slightly later, a transabdominal scan over the belly works well — your clinic may ask you to arrive with a comfortably full bladder, since fluid helps the sound waves travel. Neither method is painful, though the transvaginal probe can feel briefly uncomfortable.
Reading your scan report
Your report will list the scan date, one or more measurements (CRL in millimeters, sometimes plus head circumference or femur length later on), and the gestational age the sonographer assigned. Keep a photo of this page: it is exactly what this calculator needs, and it is what your provider will reference if any later scan raises a dating question.
Worked Example
Example 1 — gestational age in weeks and days: Your scan on January 1, 2026 measured the pregnancy at 10 weeks 0 days (70 days of gestational age). Remaining days: 280 − 70 = 210.
January 1, 2026 + 210 days = July 30, 2026 — your estimated due date.
Example 2 — CRL measurement: Your scan on June 10, 2026 measured a CRL of 45 mm, which converts to about 11 weeks 3 days (80 days). Remaining days: 280 − 80 = 200.
June 10, 2026 + 200 days = December 27, 2026 — your estimated due date.
Example 3 — late first-trimester scan: Your scan on September 15, 2026 measured the pregnancy at 12 weeks 3 days (87 days). Remaining days: 280 − 87 = 193. September 15, 2026 + 193 days = March 27, 2027 — your estimated due date.
How Accurate Is It?
First-trimester ultrasound dating is the gold standard: CRL-based dating carries an uncertainty of roughly five to seven days, because embryos grow at a very consistent rate in early pregnancy. That beats LMP dating whenever the last period is uncertain or cycles are irregular — and it is why providers confirm every pregnancy this way when possible. Second-trimester scans are a step less precise (about 10 to 14 days), since babies start growing at individual rates, and third-trimester measurements are too variable for dating at all.
One honest caveat: even the best dating method produces an estimate, not a prophecy. Studies estimate only a small share of babies arrive on their exact due date. Use your result to anchor screening schedules and follow along in our week-by-week pregnancy guide, while remembering the delivery window stretches several weeks on either side.
Accuracy also depends on which trimester the scan belongs to. A CRL-based first-trimester date is the one providers trust to set the official EDD. A second-trimester scan can still confirm or modestly adjust the date, but with wider uncertainty — and by the third trimester, measurements tell your provider about growth, not age. If you have scans from more than one trimester, the earliest dating scan is the one your provider will weigh most heavily.
When to Talk to Your Doctor
Your provider uses the dating scan to set the official due date, so bring your scan report and any LMP or ovulation dates to your first appointment and ask which date they are using going forward. Contact your provider promptly if you experience vaginal bleeding, severe abdominal pain, or dizziness in early pregnancy — and if a scan ever shows a large gap between measured age and your expected dates, ask what the discrepancy means for your care plan rather than worrying in silence.
One last practical point: save your dating scan report somewhere you can find it. Later in pregnancy, a new sonographer or a different clinic may ask for your original dating measurements, and having the report on hand avoids repeat scans and keeps every provider working from the same calendar.
This calculator provides estimates for educational purposes and is not medical advice. Always confirm with your healthcare provider.
Frequently Asked Questions
Common questions answered by our medical team
Take the scan date, note the gestational age the sonographer measured (in weeks and days), and add the remaining time to reach 40 weeks. In other words: due date = scan date + (280 days − gestational age in days). A scan measuring 10 weeks 0 days means 70 days have passed and 210 remain, so the due date is 210 days after the scan.