The researchers based their findings on 570 only children from among over 3000 born at one Dutch hospital between 1975 and 1978. Details of the smoking habits of the mothers during pregnancy and the results of maths, spelling, and reading tests taken by the children at the age of 9 were analysed.
How the mothers chose to feed their children was also recorded on discharge from hospital and when their children were 9 and again when they were 25, to ensure that recall was reliable. Children who were breastfed for only a couple of weeks, or who were fed both breast and formula milk, were not included in the final analysis.
Only those children whose mothers had smoked during pregnancy and who had been bottle fed performed poorly on the school tests.
Mothers who breastfeed their children might differ from women who don't in some way that has not been accounted for, say the authors, in a bid to explain their findings. Or there may be some psychological aspects of breastfeeding that might influence a child's cognitive development, they suggest.
Alternatively, they suggest that the long chain polyunsaturated fatty acids contained in breast milk may promote brain development and so counteract the harmful effects of smoking on fetal development. And they conclude that expectant mothers who smoke should be encouraged to breastfeed.
[Can breast feeding modify the adverse effects of smoking during pregnancy on the child's cognitive development? 2003; 57: 403-4]