International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown-rump length in the first trimester

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Objectives: There are no international standards for relating fetal crown-rump length (CRL) to gestational age (GA), and most existing charts have considerable methodological limitations. The INTERGROWTH-21st Project aimed to produce the first, international standards for early fetal size and ultrasound dating of pregnancy based on CRL measurement. 

Methods: Urban areas in eight geographically diverse countries that met strict eligibility criteria were selected for the prospective, population-based recruitment, between 9+0 to 13+6 weeks of gestation, of healthy well-nourished women with singleton pregnancies at low risk of fetal growth impairment. GA was calculated on the basis of a certain LMP, regular menstrual cycle and lack of hormonal medication or breastfeeding in the preceding two months. CRL was measured using strict protocols and quality control measures. All women were followed up throughout pregnancy until delivery and hospital discharge. Neonatal and fetal deaths, severe pregnancy complications and congenital abnormalities were excluded. 

Results: A total of 4,607 women were enrolled in the Fetal Growth Longitudinal Study (FGLS), one of the three main components of the INTERGROWTH-21st Project, of whom 4,321 women had a live singleton birth in the absence of severe maternal conditions or congenital abnormalities detected by ultrasound or at birth. The CRL was measured in 56 women at <9+0 weeks of gestation, resulting in 4,265 women who contributed data to the final analysis. The mean CRL and standard deviation (SD) increased with GA almost linearly. Their relationship to GA is defined by the two equations: Mean CRL (mm) = -50.6562 + 0.815118*GA + 0.00535302*GA2, and SD of CRL (mm) = -2.21626 + 0.0984894*GA, where GA is expressed in days. The formula for GA estimation is defined by the two equations: GA (days) = 40.9041 + 3.21585*CRL0.5 + 0.348956*CRL, and SD of GA (days) = 2.39102 + 0.0193474*CRL, where CRL is expressed in mm. 

Conclusions: We have produced international prescriptive standards for early fetal linear size and ultrasound dating of pregnancy in the first trimester that can be used throughout the world. 

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