Main Findings
Using data from a large, prospectively enrolled cohort of nulliparous
participants, we derived a sex-specific fetal growth standard that
resolves the sex disparity in SGA and LGA created by the sex-neutral
standard. The sex-specific standard identified infants labeled SGA by
the Hadlock sex-neutral standard who were not at increased risk of
morbidity but who did experience more interventions, suggesting that it
may be reasonable to safely consider them as normally-grown. It also
identified a group of otherwise unrecognized LGA infants who were at
increased risk of cesarean delivery for arrest of dilation, cesarean
delivery for arrest of descent, and shoulder dystocia, suggesting that
the sex-neutral standard under-recognizes LGA.
Overall, the sex-specific standard labeled fewer newborns as SGA and
more newborns as LGA than the sex-neutral standard. This is because the
sex-neutral standard predicts larger fetal/newborn size at the end of
pregnancy than the sex-specific nuMoM2b standard (Figure 2). It is
unclear why the Hadlock standard predicts larger size at birth than term
birth weights from our cohort since term birth weight-derived curves are
generally similar to ultrasound-derived fetal growth curves at
term.12, 18, 19 However, our new standard is
consistent with other newer standards, which also predict smaller size
than Hadlock.20, 21