I was recently on a non-BA flight when a pregnant woman was moved from an exit row. She was furious because she had paid extra for the seat and wanted a refund before she would move. They told her it was her own fault for selecting it in the first place and to either move seat or be removed from the plane. After a bit more huffing and puffing she moved. I don't know how they knew she was pregnant, it wasn't particularly obvious.