OBJECTIVES:To compare head circumference (HC) in neonates treated for neonatal abstinence syndrome (NAS) with control neonates without antenatal opioid exposure. METHODS:Our prospective cohort study ran from April 1, 2014, through December 31, 2016. Newborns treated for NAS delivered from well-dated pregnancies ≥34 weeks' gestation were compared with newborns who were nonopioid exposed and matched for race, parity, mode of delivery, and gestational age. All mothers underwent serial antenatal urine drug testing. A minimum of 754 study participants were needed (377 in each group) to demonstrate an increase in the proportion of newborns with HCs less than or equal to the 10th percentile from 10% in controls to a minimum of 20% in NAS newborns with 90% power. RESULTS:A total of 858 neonates were enrolled (429 NAS cases and 429 controls). Mean HC for cases was 33.04 cm (±1.9 cm) compared with 33.99 cm (±2.0 cm) for controls (P < .0001). Among the 429 NAS cases, the mothers of 372 (87%) were on opioid medicationassisted treatment. For NAS cases, 30.1% (95% confidence interval: 25.8%-34.7%) had an HC less than or equal to the 10th percentile (129 of 429 neonates), and 8.2% (95% confidence interval: 5.8%-11.2%) had an HC less than or equal to the third percentile (35 of 429 neonates). Multivariate analysis was used and determined that only chronic opioid use during gestation resulting in a neonate who was NAS treated was a significant risk factor for the observed smaller HC. CONCLUSIONS: Chronic opioid use during pregnancy sufficient to cause NAS was associated with smaller HCs at birth. Most mothers were on opioid agonist medication-assisted treatment, which is currently the recommended treatment option during pregnancy.

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