Introduction
Ultrasonographic assessment of fetal growth in the antenatal period is commonplace in the United States.1-3 Evaluation typically consists of calculation of estimated fetal weight (EFW) and comparison against a population average to generate a percentile value, with percentiles <10th considered small for gestational age (SGA) and >90th as large for gestational age (LGA).2-4 Accuracy of prediction of morbidity from both abnormal fetal growth remains poor, probably due to the assumption inherent in our current approach that all fetuses share a similar growth potential.5, 6
Male newborns have long been recognized to be larger than female newborns of the same gestational age, such that neonatal growth charts in the United States are sex-specific.7-9 Despite this, intrauterine growth charts remain sex-neutral.10-13 This is true even for growth charts that were developed in the era when fetal sex is routinely visible on prenatal ultrasound.10, 12 A prior analysis found that the Hadlock standard was twice as likely to consider female fetuses as being <10th percentile compared with male fetuses, even though female fetuses had significantly lower morbidity than male fetuses.14, 15 Population fetal growth standards that do not account for fetal sex, such as the Hadlock standard, may generate disparities in diagnoses of abnormal growth between fetal sexes that may not be justified by morbidity. Given the knowledge of sex differences in fetal growth and the routine prenatal assessment of fetal genitalia,1 the lack of investigation into sex-specific intrauterine growth standards represents an important gap in both research and clinical practice.
Therefore, our objectives were: (1) to derive a prescriptive sex-specific fetal growth standard; (2) to compare metrics of clinical outcomes and management according to growth status using sex-neutral versus sex-specific growth standards in an unselected cohort.