In the bikini photograph it may look like she is pushing her stomach out but if you look for where her waistline should be, there isn't one. If she wasn't pregnant, a woman of her age, slim physic and physical fitness levels would have one. If you think you do see a waistline, it isn't, it's the angle of the photo and the curve of her back.
As for the supposed prosthetic bump, what nonsense. The dress she is wearing will clearly be a fabric that has a percentage of elasticity. When she is standing the dress will sit against her body, accentuate her bump and flatter her fuller figure, when she sits it will naturally stretch and accommodate. Fabric without elasticity will leave visible signs of pulling and leave creases when stretched. The fabric has been cut on the bias, which means there is added stretch and provides better fluidity, note how the fabric around her bump droops forward, that is the fluidity of cutting on the bias. As Beyonce has an image to uphold she has to take care of how she looks at all times when in public. As for the supposed bump collapsing down, do a little test, stand up then lean forward slightly as you go to sit back down and see where your middle section goes, it gives an illusion of sinking back when in fact it doesn't, it literally is just being squashed down. Even for a woman that is 5 months pregnant who is taking care of herself so she can get back to her pre-pregnant figure will not necessarily look pregnant. She isn't exactly huge pregnancy wise, most women aren't at this stage. It is the dress' fabric content that is accentuating her figure and being misleading.
Finally, look at her face and breasts, she has put weight on in these areas. The face, especially around the jaw line is something you cannot fake weight wise.
As an aside, I've been making clothing for over 20 years so I'm fairly experienced in my knowledge of fabrics and how they work.