Abstract: BACKGROUND:Pregnant mothers with opioid dependency commonly receive maintenance treatment of opioid (OMT), either as buprenorphine (BMT) or methadone maintenance treatment (MMT). We investigated, whether OMT adversely affects standardized neonatal anthropometric outcomes and whether BMT is potentially safer than MMT in this regard. METHODS:Retrospective chart review of mother infant dyad, with and without OMT. Infant’s absolute and standardized (z-score) anthropometric outcomes at birth were first compared, between OMT and control group (negative meconium drug screen), and then between BMT and MMT group. These outcomes were also compared between infants who did or did not require treatment after birth for neonatal abstinence syndrome (NAS). RESULT:A total of 1479 participants with MDS were included [Control = 1251; OMT = 228 (MMT = 181; BMT = 47)]. Both the z-scores of birth weight (BW) and head circumference (HC) was lower in OMT group (p < 0.001). Among the OMT group, GA at delivery was slightly higher in the BMT group (p = 0.05). There was an inverse correlation between maternal dose at the time of delivery and anthropometric z-scores in the BMT group, mainly in female infants (BW: p = 0.006; HC: p = 0.003). Furthermore, In BMT group, infants with lower HC were more likely to require treatment for NAS (p = 0.01). CONCLUSION:HC and BW when comparing Z-scores were not different between MMT and BMT. High maternal dosing of buprenorphine is associated with lower BW and HC Z-scores but dose effect is not seen with methadone. In addition, there seems to be an association between NAS severity and HC, especially in the BMT group.