Crown-Rump Length Growth Rate in Early Pregnancy
Crown-rump length increases by approximately 1 mm per day during the first trimester, with this linear growth pattern maintained from approximately 6 weeks through 14 weeks of gestation. 1, 2
Daily Growth Rate
The mean CRL increases almost linearly with gestational age throughout the first trimester, with the relationship described by the equation: mean CRL = -50.6562 + (0.815118 × GA) + (0.00535302 × GA²), where GA is in days and CRL in mm 1
This translates to an average daily increase of approximately 1 mm per day during the 6-14 week window 1, 2
The linear growth pattern is remarkably consistent across geographically diverse populations, as demonstrated by the INTERGROWTH-21st international standards 1
When Growth Rate Changes
The growth rate does NOT significantly slow down during the first trimester - the relationship between CRL and gestational age remains nearly linear from 6 to 14 weeks 1, 2
After 14 weeks of gestation, CRL is no longer the primary biometric measurement used, as other parameters (biparietal diameter, femur length, abdominal circumference) become more reliable for dating and growth assessment 3
The quadratic component in the growth equation is minimal, indicating that any deceleration in the first trimester is negligible for clinical purposes 1, 2
Clinical Implications for Growth Assessment
Abnormally slow CRL growth between serial measurements in the first trimester is a significant predictor of adverse outcomes, including increased risk of miscarriage (7.1% vs 0.9% in normal growth), trisomy 18 (4.8% vs 0.1%), and small-for-gestational-age infants (12.8% vs 3.6%) 4
When measuring CRL at 9 weeks and again at 11-13 weeks, fetuses with slower-than-expected growth (short CRL at the second measurement based on dating from the first) have substantially worse prognosis 4
A minimum interval of 7-14 days between measurements is recommended before concluding abnormal growth, as measurement error of even 2 mm can significantly impact percentile classification 5, 3
Critical Measurement Considerations
Measurement error of ±2 mm is common in practice and can shift an EFW from the 10th percentile to the 5th or 20th percentile, making precise technique essential 5
CRL should be measured along the longest axis of the embryo with the fetus in neutral position (neither hyperflexed nor hyperextended) 3
The standard deviation of CRL measurements increases with gestational age according to: SD of CRL = -2.21626 + (0.0984894 × GA) 1