also: マタニティーマーク
pregnancy badge; baby on board badge
A badge or tag worn by pregnant women in Japan to indicate pregnancy, especially in public transport, to encourage consideration from others.
wasei-eigo (Japanese-coined English), from maternity mark
I gave up my seat to a person wearing a pregnancy badge on the train.
マタニティマークは、妊娠初期でも周囲に気づいてもらいやすくするためのものです。
The pregnancy badge is meant to help others notice even in early pregnancy.