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Substitute methadone prescribing in pregnancy is associated with impaired fetal head growth

Background Exposure to illicit drugs in utero is associated with restricted fetal growth. The magnitude of this effect is not clear as data are commonly confounded by cigarette smoking and social deprivation.123 The aim of this study was to determine the extent of in utero growth restriction attributable to maternal drug misuse, and to examine relative effects of maternal drug misuse upon fetal somatic and head circumferential growth. Methods Birthweight and head circumference (OFC) were documented for 366 singleton infants of drug-misusing women prescribed substitute methadone and delivered at a single hospital over a 3-year period. Administrative obstetric records of all women delivering in Scotland over 3 years were used to select controls, matching on maternal smoking, social deprivation (Scottish Index of Multiple Deprivation) and gestation. Birthweight and OFC of methadone-exposed infants were compared to those of 1956 matched infants, using regression analysis to control for sex, maternal age and parity. Findings Methadone-exposed infants were on average 351 g lighter with OFC 1.2 cm smaller than controls. Translated into z-scores, this represents a difference of 0.48 SD in birthweight and 0.58 SD in OFC. Conclusion After adjusting for major confounding factors methadone-exposed infants showed reductions in OFC that were at least as great as reductions seen for birthweight. This suggests that in utero methadone and/or other illicit drug exposure affects somatic and brain growth equally. This is in contrast to other in utero insults such as smoking, where head growth is relatively spared.