In particular, breastfeeding may be of concern because it has been shown to be associated with both child's use of antibiotics36, 37 and cow's milk allergy,
although inconsistently so.38 In a large Finnish birth cohort, the median durations of exclusive and total breastfeeding were 1.4 months and 7.0 months, respectively, and the proportion of breastfed infants decreased from 95 % at the
age of 1 month to 58 % at the
age of 6 months.39 Another
limitation is our reliance on pharmacy records, which provides only a rough estimation of drug use.