Yet another reason not to smoke

Maternal cigarette smoking during the third trimester, adjusted for the seven covariates, showed a negative association with offspring adult intelligence (P=0.0001). The mean difference between the no-smoking and the heaviest smoking category amounted to 0.41 standard deviation, corresponding to an IQ difference of 6.2 points

Researchers reviewed records from a Danish birth cohort with IQs obtained from testing when they were 18 as part of compulsory draft-board registration. The study is reported in the January issue of Paediatric and Perinatal Epidemiology (ed. Paediatric? It's a British journal. oh.)

The data is "old" with the kids being born in Denmark between 1959 and 1961.

Of course, these kind of studies can only show association, not causation. If low maternal intelligence is both heritable (either due to genetics or environment) and predicts maternal smoking (seems to) it could cause the association. The authors note that negative effects of smoking during pregnancy were not widely appreciated in Denmark at that time (ed if your IQ was 6.2 points higher you would have used "deleterious!" Shut up). About half the women smoked, and even quote a contemporary references endorsing smoking during pregnancy as a remedy for constipation.