Objectives: To construct normal ranges for embryonic crown-rump length (CRL), heart rate (HR), gestational sac diameter (GSD) and yolk sac diameter (YSD) at 6–10 weeks of gestation. Methods: We examined 4,698 singleton pregnancies with ultrasound measurements of CRL, HR, GSD and YSD at 6–10 weeks and CRL at 11–13 weeks resulting in the live birth after 36 weeks of phenotypically normal neonates with birth weight above the 5th centile. Gestational age was derived from CRL at the 11- to 13-week scan using the formula of Robinson and Fleming. Regression analysis was used to establish normal ranges of CRL, fetal HR, GSD and YSD with gestation, and fetal HR, GSD and YSD with CRL. Results: At 6–10 weeks there were significant quadratic associations between CRL, GSD, YSD and gestation and between HR, GSD, YSD and CRL, and a cubic association between HR and gestation. The estimated gestation from CRL was the same as that of Robinson and Fleming for a CRL of 10.2–36.5 mm, but the formula of Robinson and Fleming underestimated the gestation by 1 day for a CRL 7.4–10.2 mm and this increased to 9 days for a CRL of 1 mm. Conclusion: This study established normal ranges for early pregnancy biometry.

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