"Any regular, daily antenatal opioid exposure (e.g., morphine, codeine, oxycodone, methadone, or buprenorphine) can produce neonatal withdrawal, also known as neonatal abstinence syndrome [NAS]. Estimates show that up to 96% of infants display withdrawal symptoms, and a smaller proportion require pharmacotherapy.4,68,116,117,120,121 NAS is characterized by respiratory, gastrointestinal, central nervous system, and autonomic symptoms (Table 6). Onset of withdrawal symptoms is dependent on the opiate’s halflife; the longer the half-life, the later the onset of withdrawal. Heroin-exposed infants may demonstrate symptoms within 24 hours of birth. In comparison, methadone-maintained infants have a delayed presentation at more than 24 hours, usually within 48 to 72 hours after birth and at up to 4 weeks of age.122 The length of monitoring is based on the specific drug exposure. Treated neonatal withdrawal has not been associated with any long-term complications."
Wong, Suzanne; Ordean, Alice; Kahan, Meldon, "Substance Use in Pregnancy," Society of Obstetricians and Gynaecologists of Canada: Ottawa, Ontario: April 2011.