HEALTH · DUE DATE FROM ULTRASOUND
Due Date from Ultrasound Calculator
Calculate your estimated due date from a first-trimester ultrasound crown-rump length (CRL) or gestational age measurement. Uses the Hadlock 1992 CRL formula per ACOG guidance.
Hadlock 1992 formula: GA (days) ≈ CRL × 0.9 + 25. Best accuracy for CRL 10–84 mm.
About This Calculator
An ultrasound scan between 7 and 14 weeks provides one of the most accurate ways to estimate your due date, especially when your last menstrual period is uncertain. This calculator converts your crown-rump length (CRL) measurement into a gestational age and estimated due date using the Hadlock 1992 formula — the same reference used in most clinical ultrasound software.
How It Works
Choose CRL mode if you have a millimeter measurement from your scan report, or GA mode if your sonographer reported a gestational age directly. Enter your scan date, then enter the CRL in millimeters (10–84 mm, approximately 6–14 weeks) or the gestational age in weeks and days. The calculator uses EDD = scan date + (280 − gestational age at scan).
The Formula
EDD = scan_date + (280 − GA_at_scan_days)
- GA_at_scan_days
- gestational age in days at the time of the ultrasound scan
- scan_date
- date of the ultrasound examination
Frequently Asked Questions
- When is ultrasound dating most accurate?
- Ultrasound dating is most accurate between 8 and 13 weeks + 6 days (crown-rump length ≥ 10 mm, ≤ 84 mm). ACOG recommends using CRL dating during this window because a discrepancy of more than 5–7 days from the LMP-based EDD generally warrants updating the due date to the ultrasound estimate. After 14 weeks, other biometric measurements (biparietal diameter, femur length) are used instead of CRL.
- What is a crown-rump length (CRL)?
- CRL is the distance from the top of the embryo's head (crown) to the bottom of its buttocks (rump), measured in millimeters on ultrasound. It is the most reliable single measurement for first-trimester dating. A CRL of 10 mm corresponds to roughly 6 weeks 2 days; 84 mm corresponds to approximately 14 weeks 3 days.
- What if my LMP-based date and ultrasound date disagree?
- ACOG guidance states that if the ultrasound date differs from the LMP estimate by more than 5 days (before 9 weeks) or 7 days (9–14 weeks), the ultrasound date should replace the LMP estimate as the official EDD. Your care provider makes this determination; this calculator is for informational reference only.