Why are moms always so happy (relieved?) to hear of other women getting pregnant?

My sister is pregnant and women are coming out of the woodwork to tell her things like "Oh, your face is already getting fat" Or "I can't wait to see you when you start waddling" These women are eerily excited about the idea of my sister losing her body. It is disturbing, given how every mom thinks her child is the best thing she's ever done.