Well, not that I'm analysing her boobs or anything but...,
This series would be her 4th child (2 prior to coming mistress en titre with her husband and her second with Louis, although they had more!). Her boobs didn't look like that in the last series. I think this is body doubling with clever suits to show she is very, very pregnant. It's not obvious in the dresses, so although the nudity is quite gratuitous, I think it serves to show she is used to being pregnant and that she is far gone and it hasn't altered his desire for her (which is also factually correct)