"There are numerous benefits of methadone use during pregnancy, including improved prenatal care,12,107–109 longer gestation,50,110 higher birth weight,111,112 and increased rates of infants discharged home in the care of their mothers.4,12,18,49,101,108,113–118 Although infants of methadone-treated women tend to be smaller (lower birth weight, length, and head circumference) than drug-free controls, studies have shown a catch-up of growth by 12 months of age.118,119"

Source

Wong, Suzanne; Ordean, Alice; Kahan, Meldon, "Substance Use in Pregnancy," Society of Obstetricians and Gynaecologists of Canada: Ottawa, Ontario: April 2011.