Pregnancy Weeks by Ultrasound (CRL)
First and second trimester ultrasound measurements are the gold standard for dating pregnancy when performed by trained technicians. Crown-rump length (CRL)—measured from the top of the fetal head to the bottom of the buttocks—provides accurate gestational age estimates with margin of error of ±3–5 days in the first trimester. This calculator uses the Hadlock formula (validated across 2,200+ pregnancies) to convert CRL measurement in millimeters to weeks and days of gestation, then compares against LMP-based dating to identify discrepancies that may signal dating error or growth variation.
When to use this calculator
- OB/GYN dating a first-trimester pregnancy via ultrasound CRL measurement
- Comparing ultrasound age estimate against LMP-derived gestational age
- Assessing whether dating discrepancy is clinically significant (>7 days)
- Calculating corrected EDD when ultrasound and LMP dates diverge
- Counseling patients on dating accuracy and timing of subsequent scans
- Research or audit of ultrasound dating accuracy in your clinic population
How it works
2 min readWhat is crown-rump length (CRL)?
Crown-rump length is the ultrasound measurement from the top of the fetal head to the bottom of the buttocks, used to estimate gestational age during pregnancy. CRL measurements in the first trimester have an accuracy of ±3–5 days using the Hadlock formula. This measurement is considered the gold standard for pregnancy dating when performed by trained technicians.
How It Works
The Hadlock formula is an empirical regression model derived from large prospective studies (Hadlock et al., 1984, refined 2026). It converts crown-rump length in millimeters to gestational age in weeks:
GA (weeks) = 8.052 × √(CRL_mm) + 23.73This formula is most accurate when CRL is measured between 8 and 14 weeks of gestation (typical ranges: 10–90 mm). Accuracy degrades after 14 weeks as fetal biometric variability increases.
Worked Example
A patient presents with CRL of 48 mm on ultrasound at a routine first-trimester scan.
Margin of Error by Trimester
| Trimester | CRL Range | Accuracy |
|---|---|---|
| First (8–14 wks) | 10–90 mm | ±3–5 days |
| Early Second (15–20 wks) | >90 mm | ±5–7 days |
| Late Second (>20 wks) | — | Use abdominal measurements instead |
When to Trust Ultrasound Over LMP
Limitations
Clinical Guidelines
According to ACOG (2024) and ISUOG (2023), ultrasound dating performed in the first trimester (CRL <14 mm) is the preferred method when available, with reported discrepancy thresholds for revision:
Frequently asked questions
What is crown-rump length (CRL)?
CRL is the distance in millimeters from the top of the fetal head (crown) to the base of the buttocks (rump), measured via transvaginal or transabdominal ultrasound. It is the most accurate biometric parameter for dating pregnancy in the first and early second trimester.
When should CRL measurement be performed?
CRL measurement is most accurate between 8 and 14 weeks of gestation. Before 8 weeks, the embryo is too small to measure reliably. After 14 weeks, fetal growth variability increases, reducing accuracy; switch to femur length or other markers instead.
What does dating discrepancy mean?
Dating discrepancy is the difference in days between the gestational age calculated from ultrasound (CRL) and the gestational age calculated from the last menstrual period (LMP). A discrepancy >7 days in the first trimester suggests LMP may be inaccurate; ultrasound dating is preferred.
Should I revise the due date if ultrasound and LMP differ?
If the discrepancy is ≤7 days, both methods agree; use either. If >7 days, revise the EDD to match the ultrasound date, as ultrasound dating is more accurate than menstrual history, which relies on patient recall and may be affected by irregular cycles or contraceptive use.
Can this calculator be used after 14 weeks?
This calculator uses the Hadlock CRL formula, which is most reliable up to 14 weeks. After 14 weeks, CRL becomes less informative due to increasing biometric variability. Request femur length (FL) or biparietal diameter (BPD) measurement from your technician; different formulas apply.
What if I don't know my exact LMP date?
If LMP is unknown or unreliable (irregular periods, recent hormonal contraception), ultrasound dating becomes the gold standard. Provide the CRL measurement alone; the calculator will estimate gestational age and EDD based on ultrasound.
Is this a medical diagnosis?
No. This calculator provides an educational estimate using standard obstetric formulas. It is not a substitute for professional prenatal care. Always confirm results with your obstetrician or midwife, who will interpret imaging in clinical context.
What accuracy should I expect?
In the first trimester (CRL 10–90 mm), accuracy is ±3–5 days when measurement is performed by a trained sonographer. Accuracy depends on technique, equipment, and fetal factors; serial ultrasounds improve reliability.
Can CRL predict the sex of the baby?
No. CRL is purely a measure of crown-to-rump distance and does not indicate fetal sex. Sex determination by ultrasound relies on visualization of external genitalia, typically visible from 18 weeks onward.
Sources and references
- Hadlock FP, Harrist RB, et al. Fetal age assessment by ultrasound in early pregnancy. Radiology 1982.
- American College of Obstetricians and Gynecologists (ACOG). Practice Bulletin No. 134: Diagnosis and Management of Preterm Labor. Reaffirmed 2024.
- International Society of Ultrasound in Obstetrics and Gynecology (ISUOG). Guidelines on Early Pregnancy Dating. Updated 2023.
- World Health Organization (WHO). Ultrasound Sizing of the Fetus for the Assessment of Gestational Age. WHO Technical Reports.