Why? Some women feel like absolute shít during pregnancy (more often than not during the first trimester when they are unlikely to be showing) and feel a little bit awkward about asking for a seat.
You think sitting down takes away morning sickness, Monty? Nah, they're just being drama queens looking for a seat.
Mrs WES rode the Waterloo and City line until 8 months pregnant and never once wore one of those buttons. Proper woman.