My mother always told me she and my grandmother loved drinking a Trappist beer when they were breastfeeding, she said that it was nurturing for new moms and that in the old days the nurses would actually bring a beer to the mothers to stimulate the lactation process.
But it isn't just a myth, if a nursing mother drinks a good old pint of beer, the yeast and hops in it will help increase her milk supply. Hops are also calming, so good for the new mom. Brewers yeast is also taken as a supplement to boost the milk supply by mothers who do not enjoy the taste of a lovely beer.
Trappist is a Belgian beer but I think Stout is the prefect beer for this recipe as I've heard stories about mothers receiving a Stout when they have given birth, a Nourishing Stout would have been better but sadly those haven't been brewed commercially for decades. Milk Stout is called that way because it used to contain lactose, a sugar derived from milk. Lactose doesn't only add sweetness to the beer, it also adds calories which is why together with the yeast and hops in the beer it was given to lactating mothers. Although Milk and Nourishing Stouts only became popular after the First World War, the usage of lactose and the mentioning or illustrating of it on the beer labels was forbidden after the Second World War due to rationing.
The only surviving Milk Stout is Mackeson's, I came across it by accident when I was at Tesco's a few months ago, the can still shows a milk churn that has been Mackeson's trademark since it was first brewed in 1907 at the Mackeson's brewery in Hythe, Kent. Mackeson's is now brewed by InBev so I doubt that there is still any lactose in the beer today.