Carbon monoxide (CO) is a highly toxic gas which, at high levels, can kill. Such lethal events are often accidental. When a pregnant women is involved in such an acute CO poisoning episode it can have demonstrable deleterious effects on the unborn child, with increased chance of stillbirth or the birth of a child with motor and/or mental abnormalities. Post-mortem examination of these stillbirths has indicated that the major effect of the acute CO event was on the central nervous system. This raises the question as to what effect chronic exposures might have on the brain of the unborn fetus or developing child. This project will use data already collected by ALSPAC: Prenatal and early life exposure to CO from domestic heating and cooking and proximity to busy roads.