"Infants born to mothers who use opiates do not have birth defects due to the effects of the opiates, but do have impaired growth, smaller head size, and significant neurobehavioral dysfunction due to withdrawal. Women who use opiates usually use other illegal and legal drugs, are poorly nourished, and frequently have other health problems, all of which contribute to adverse newborn outcomes, especially poor growth and small head size."
Source
Zuckerman, Barry, "Drug-Exposed Infants: Understanding the Medical Risk," The Future of Children, Spring 1991, Volume 1, Number 1, p. 28. Citing Institute of Medicine, Preventing Low Birthweight: Summary, National Academy Press, Washington, D.C., 1985.