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We would like to thank Dr Aarstad1 for his interest in our work. We noted in the ‘Results’ section2 that the rates of cell phone use increased during the time of our study. For instance, the percentage of mothers using a cell phone during pregnancy increased from 21% to 46% from the birth years 1998 to 2001. And as stated in our conclusion, we do not rule out the possibility that uncontrolled confounding may play a role in our observed associations.
Examination of heterogeneity over time within social–occupational status is presented …
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