Maternity mystery

I bet that’s got your attention. However, no, I do not have any news to announce (in fact, if I did have news of that nature, it would REALLY be a mystery, especially to me … I won’t go into any more details but refer to the post where I mentioned that at the moment my cat – and only my cat – shares my bed). So getting back to the point… a few months ago, I was foraging in an op shop (nothing unusual there) and found this sewing pattern, which is for a skirt. But not just any skirt. The back of the packet says that it’s for “ladies in waiting”, which in today’s elegant language translates as “being up the duff”. I love all retro patterns including this one which looks to be from the 50s, but I can’t for the life of me understand how it works. If it was made as a skirt to wear over something else, it would be weird, because you’d have a hole showing off your lump and highlighting what would have in those days been called your “condition”. I think it’s intended as an undergarment, from the pictures, but I can’t really see the point if a normal skirt just gets worn over the top. Can anyone enlighten me on this one?