I also found maternity and nursing clothes difficult to find. I agree with H&M but I also used Shein, Seraphine and Amazon (I know not European brands unfortunately). Or in many things, I just bought a larger size of normal clothes which worked well especially with floaty dresses
I used H&M too and Amazon for maternity leggins đ used my normal clothes for the top part
Calzedonia sells maternity comfy trousers and leggins
Zalando and ASOS have some good options, as well as Vinted!
I have found the H&M mama like to be the best option in Italy. Nicer options than Prenatal, definitely!