Ultrasound Dating FAQs
Detailed answers to frequent clinical questions about crown-rump length measurements, gestational age calculation, and scan reports.
Crown-Rump Length (CRL) Basics
What is Crown-Rump Length (CRL)?
Crown-Rump Length (CRL) is the ultrasound measurement of human embryo or fetus from the top of the head (crown) to the bottom of the buttocks (rump), excluding limbs and the yolk sac. It is the international gold standard for pregnancy dating between 6 and 14 weeks of gestation.
Why is CRL considered more accurate than Last Menstrual Period (LMP)?
LMP dating assumes a regular 28-day cycle with ovulation occurring precisely on cycle day 14. However, ovulation timing varies considerably between women and even across different cycles for the same woman. In contrast, first-trimester embryonic growth rate is remarkably uniform across all humans, giving CRL dating a precision margin of ±3 to 5 days.
When is CRL measured during pregnancy?
CRL is routinely measured on the first-trimester dating ultrasound scan, typically between 6 weeks 0 days and 13 weeks 6 days (corresponding to embryonic lengths from ~5 mm to 84 mm).
Clinical Formulas & Mathematics
What formula is used to calculate gestational age from CRL?
Our calculator implements the Robinson & Fleming (1975) equation adopted as standard by the British Medical Ultrasound Society (BMUS), NICE, and The Fetal Medicine Foundation (FMF): GA (days) = 8.052 * sqrt(CRL * 1.037) + 23.73. We also provide the Hadlock (1992) regression correlation and the INTERGROWTH-21st standard.
How is the Estimated Due Date (EDD) determined?
Human gestation spans 280 days (40 weeks 0 days) from the first day of the last menstrual period. When CRL determines gestational age (e.g. 9w 2d = 65 days), the remaining days until delivery are 280 - 65 = 215 days. Adding 215 days to the ultrasound scan date yields your Estimated Due Date (EDD).
What is the confidence interval or margin of error?
For CRL measurements under 50 mm, the 95% confidence interval is approximately ±3 to 4 days. For CRL between 50 mm and 84 mm, the margin is approximately ±5 days. This is far more precise than second-trimester dating (±7 to 10 days) or third-trimester dating (±14 to 21 days).
Discrepancy with Menstrual Dates (ACOG)
What happens if my CRL date does not match my period date?
Under ACOG Committee Opinion No. 700 guidelines: if gestational age is up to 8w 6d and the ultrasound CRL differs by more than 5 days from LMP, the due date is redated to the ultrasound. If gestational age is between 9w 0d and 13w 6d and the difference is greater than 7 days, the due date is also redated to ultrasound.
Why would my baby measure smaller than my period date?
The most common reason is delayed ovulation or implantation. For instance, if you ovulated on cycle day 21 instead of day 14, the embryo will measure approximately 7 days younger than your calendar period date.
Ultrasound Milestones & Follow-Up
At what CRL can a heartbeat be seen?
A rhythmic embryonic cardiac flicker is usually visible on transvaginal ultrasound when the embryonic pole reaches a CRL of 2 to 4 mm (around 5.5 to 6.0 weeks). By CRL of 5 mm, cardiac pulsations should be definitively documented.
What is the Nuchal Translucency (NT) scan window?
The first-trimester combined screening / Nuchal Translucency (NT) scan must be performed strictly when CRL is between 45 mm and 84 mm (11 weeks 0 days to 13 weeks 6 days).
Why does ultrasound stop measuring CRL after 14 weeks?
Beyond 84 mm (~14w 0d), the fetus curls, flexes, and straightens its spine freely, making full crown-to-rump measurements imprecise. Sonographers transition to measuring Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL).