It can be either positive or negative. For some mums, breastfeeding can help reduce the chance of postnatal depression or shorten how long it lasts. For others it may cause a lot of stress and anxiety. It can sometimes be helpful for those mums to stop breastfeeding.
It is very important for both the mum and her partner to learn to recognise the symptoms of antenatal and postnatal depression so that you can ask for help as early as possible. The sooner the depression is identified and the right sort of support given, the earlier you can start to recover.
Antenatal and postnatal depression are both very treatable.
Source: https://www.breastfeeding.asn.au/resources/pnd-and-breastfeeding