The gestational age from the measurement of CRL is derived from the formula reported by: Robinson HP, Fleming JE. A critical evaluation of sonar crown rump length measurements. BJOG 1975; 182: 702–710 .

The gestational age from the measurement of HC is derived from the formula reported by: Snijders RJ, Nicolaides KH. Fetal biometry at 14-40 weeks' gestation. Ultrasound Obstet Gynecol 1994; 4: 34-48 .