It isn't about wearing a sack, or about her being pregnant imo - leggings without a top long enough to cover groin/arse at least partially look hideous.
It is perfectly possible to be comfortable in pregnancy without looking scary - especially if you can afford to buy decent clothes (I have a handful of outfits so am jealous of anyone who could potentially buy a "maternity wardrobe").
I do agree with Rinc though on the boakiness