A fetal growth standard derived from multiple modalities

Abstract

Objective: To create a fetal weight growth standard from published data on ultrasound-dated pregnancies.

Methods: A fetal growth standard was calculated from published birth weight data, sonographic weight standards, and one MRI study. The birth weights from the East Midlands Obstetric Database were modified by an incremental function to compensate for the lower weights of preterm infants. Published sonographic and MRI standards, and the modified birth weight curve were transformed to yield fractional growth curves that express fetal size as a proportion of the expected term weight. These three curves were then averaged, and the standard deviation was taken as 12% of the median. Curves for Europeans and Chinese were then generated.

Results: The average curve for fetal weight is a virtually linear function of gestational age, with a weekly weight gain of 5.2% of the expected term birth weight.

Conclusions: Ultrasound-derived fetal growth curves are subject to bias inherent in weight estimation formulae. A standard derived from multiple modalities is likely to be a more robust estimate of normal fetal growth.