In the Australian interview it is her facial expression that gives the game away. She has a look of near-panic when the dress pops out the way it does and it is obvious that she is thinking "SHIT".
And even if it is a maternity dress, it really has no reason to be edging forward at such a sharp angle. It is quite clear to me that there is something pertruding from underneath the dress like padding of some sort.
This. There are moments in life when you just can't stop making expressions, as much as you want to - a bit like a dog can't voluntarily stop its tail from wagging. Beyonce's look at that moment was involuntary and panic stricken. There was no chance of her showing her knickers how she was sitting/sat so we can rule that anxiety out. She doesn't even try and sit like a 5 month pregnant woman would sit, she then proceeds to cross her legs (wouldn't happen) and even more damning (and I'm amazed this hasn't cropped up more in the discussion), she then proceeds to bend forward without opening her legs to kiss the interviewer. If the size of the bump she displayed when walking onto the set was indeed solid then to do what she did then would be impossible.