Risk factors for preterm birth among gravid individuals receiving buprenorphine for opioid use disorder

Background: Opioid use disorder (OUD) has dramatically increased over the last few decades, with 11.5 million American misusing opioids in 2016. Untreated OUD in pregnancy is associated with unique adverse obstetric and perinatal outcomes including insufficient prenatal care, preterm birth (PTB), fetal growth restriction, fetal demise, and placental abruption . The mainstay treatment for OUD management in pregnancy is medication for opioid use disorder (MOUD) including methadone or buprenorphine. The association of PTB and opioid use in pregnancy has been described for over 50 years, and efforts to significantly eliminate this risk are challenged by the many confounding risks described above. When comparing rates of PTB in individuals with OUD on methadone vs buprenorphine. Buprenorphine has been associated with overall lower PTB than Methadone by almost 50 %.

Objective: Pregnancies complicated by opioid use disorder are at an increased risk for preterm birth, defined as delivery <37 weeks' gestation. Limited literature is available on the prevalence and risk factors for preterm birth in pregnancies complicated by opioid use disorder maintained on buprenorphine. Therefore, we sought to determine the rate of preterm birth and risk factors for preterm birth in this population.

Study design: We performed a retrospective cohort study of pregnant individuals with singleton gestations receiving buprenorphine for opioid use disorder, who delivered at a tertiary academic medical center between July 1, 2013 and June 30, 2018. Individuals who had at least 3 visits to our colocated clinic were included in the analysis. Patients were divided into 2 groups: the preterm group for patients who delivered at <37 weeks of gestation and the term group for those who delivered at ≥37 weeks of gestation. We defined "supplements to buprenorphine" to include any illicit drugs found on antepartum urine toxicology. Variables evaluated as potential risk factors for preterm birth included medical and infectious comorbidities and illicit polysubstance use.

Results: The overall preterm birth rate in this cohort was 22.7% (115/507). There was a nonsignificant trend toward decrease in overall preterm birth and provider-initiated preterm birth rate over the study period. No differences were found between the groups in spontaneous preterm birth rate at <34 weeks of gestation. There were no differences between the groups in the use of tobacco or alcohol, number of prenatal visits, or gestational age when prenatal care started. Individuals with preterm birth in the index pregnancy were more likely to have a history of preterm birth than individuals with term delivery (73% vs 16%; P<.01). No medical or infectious comorbidity or any specific supplement increased the risk of preterm birth. Among individuals using 0, 1, 2, or 3 or more illicit supplements in addition to confirmed buprenorphine for opioid use disorder, the preterm birth rate was 27.4% (reference), 18.0% (P=.09), 18.1% (P=.44), and 15.8% (P=.77), respectively.

Conclusion: The preterm birth rate among individuals using buprenorphine for opioid use disorder (22.7%) is higher than the national average but lower than the reported preterm birth rate in individuals using methadone for the treatment of opioid use disorder. No medical or infectious comorbidity or use of additional illicit substances increased the risk of preterm birth.