Here's a painting by Marcus Gheeraerts II called "Portrait of an Unknown Lady." It's a nice example of a royalty in a maternity dress. The woman's outfit is elegantly embellished in pearls on top of embellished lace. A structual bodice most likely provides a puffout of the sleeves. It was likely made out of silk or a fine cotton and took months to sew.
It is interesting to see a bit of uplift on the bra and the lady obviously needed a bit of extra support as is evidented by the four thin straps that likely go over the shoulders.
The Dutch style collar would have been uncomfortable but stylish for the period. The hairband pushed the hair into a heart shape. Her hair has added jewels most likely pearls and a rubies. Since there was no lipstick at the time, she would have used crushed red coral, but, seeing as the artist could make any modifications, he would have painted her lips a brighter red.
Guessing by her size, she's probably third trimester and would have had an awful time standing for such a long time. More likely is that her dress was put on a form (or a stand in person) and she sat on a chair when the time came to paint her face and hands.
The artistry is very magnificent and shows a skill of white on grey to express depth and contour. Overall, it's a really beautiful dress but probably not all comfortable. A really nice show piece.