Abstract This study aimed to identify the causal effect of
breastfeeding on postpartum depression (PPD), using data
on mothers from a British survey, the Avon Longitudinal
Study of Parents and Children. Multivariate linear and
logistic regressions were performed to investigate the
effects of breastfeeding on mothers’ mental health measured
at 8 weeks, 8, 21 and 32 months postpartum. The estimated
effect of breastfeeding on PPD differed according to whether
women had planned to breastfeed their babies, and by
whether they had shown signs of depression during pregnancy.
For mothers who were not depressed during pregnancy,
the lowest risk of PPD was found among women who
had planned to breastfeed, and who had actually breastfed
their babies, while the highest risk was found among women
who had planned to breastfeed and had not gone on to
breastfeed. We conclude that the effect of breastfeeding on
maternal depression is extremely heterogeneous, being
mediated both by breastfeeding intentions during pregnancy
and by mothers’ mental health during pregnancy. Our results