Not necessary but why not? DHA is indeed good for brain development. Of course it will not does any harm if you don’t take any supplement at all - look at how we have grown up well and healthy when our parents did not have the idea of supplements during their era :) for myself though, I’m a bit kiasu so I just take especially I hate the fishy smell and really don’t like to eat fish which is one of the source for DHA. Taking the DHA supplement at least makes me feel better that I’m providing my baby enough nutrient for her development;)