Warme zwangerschapslegging met teddy voering

When I wore it for the first time, the fitting was very tight. So I'd recommend to buy the bigger size (S - 36, M - 40), I normally wear size 38 but S size was too small, so I had to exchange to M size. The belly part was very tight that I'm only 5 months and it was too tight. But after a few times washing and wearing, the textile stretched a lot so now it gets quite loose, especially the belly part. Now I'm not exactly sure which size is better, too bad it does not stay at the same shape.