Association Between Pregnancy Complications and Small-for-gestational-age Birth Weight Defined by Customized Fetal Growth Standard Versus a Population-based Standard

Objective: To derive coefficients for developing a customized growth chart for a Mid-Western US population, and to estimate the association between pregnancy outcomes and smallness for gestational age (SGA) defined by the customized growth chart compared with a population-based growth chart for the USA.

Method: A retrospective cohort study of an ultrasound database using 54,433 pregnancies meeting inclusion criteria was conducted. Coefficients for customized centiles were derived using 42,277 pregnancies and compared with those obtained from other populations. Two adverse outcome indicators were defined (greater than 7 day stay in the neonatal unit and stillbirth [SB]), and the risk for each outcome was calculated for the groups of pregnancies defined as SGA by the population standard and SGA by the customized standard using 12,456 pregnancies for the validation sample.

Results: The growth potential expressed as weight at 40 weeks in this population was 3524 g (standard error: 402 g). In the validation population, 4055 cases of SGA were identified using both population and customized standards. The cases additionally identified as SGA by the customized method had a significantly increased risk of each of the adverse outcome categories. The sensitivity and specificity of those identified as SGA by customized method only for detecting pregnancies at risk for SB was 32.7% (95% confidence interval [CI] 27.0-38.8%) and 95.1% (95% CI: 94.7-95.0%) versus 0.8% (95% CI 0.1-2.7%) and 98.0% (95% CI 97.8-98.2%)for those identified by only the population-based method, respectively.

Conclusion: SGA defined by customized growth potential is able to identify substantially more pregnancies at a risk for adverse outcome than the currently used national standard for fetal growth.