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The Role of Preterm Birth in the Association Between Opioid Maintenance Therapy and Neonatal Abstinence Syndrome

BACKGROUND
Pregnant women treated with methadone as opioid maintenance therapy are more likely than women treated with buprenorphine to deliver preterm. Preterm birth is associated with less risk of neonatal abstinence syndrome (NAS). We sought to assess the role of preterm birth as a mediator of the relationship between in utero exposure to methadone and NAS compared with buprenorphine.


METHODS
We studied 716 women receiving methadone or buprenorphine and delivering liveborn infants at Magee-Womens Hospital, Pittsburgh, Pennsylvania (2013-15). We implemented inverse probability weighted marginal structural models to isolate the role of preterm birth (<37 weeks' gestation). Weights accounted for confounding by maternal age, race, insurance, parity, delivery year, marital, employment, hepatitis C, and smoking status.


RESULTS
Approximately 57% of the cohort were treated with methadone. Preterm birth was more common in methadone-exposed pregnancies (25% versus 14%). The incidence of NAS treatment was higher in methadone compared with buprenorphine-exposed infants (65% vs 49%), and term compared with preterm births (64% vs 36%). For every 100 infants liveborn to mothers treated for opioid dependence, there were 13 excess cases of NAS among infants exposed to methadone compared with buprenorphine (adjusted risk difference [RD] 13.3, 95% confidence interval [CI] 5.7, 20.9). Among term births, this increased to 17 excess cases of NAS in methadone- compared with buprenorphine-exposed (RD 16.7, 95% CI 9.3, 24.0).


CONCLUSION
The further increased risk of NAS associated with methadone use vs buprenorphine in term deliveries emphasises the utility of buprenorphine in clinical settings aimed at decreasing NAS.