As well as the ever-present danger of cars speeding around narrow roads and the hassle of lugging strollers up and down staircases, parents in Japan with babies in tow have long had to struggle with public restrooms the size of telephone booths.

Every parent knows that answering nature's call with a wiggly kid in your arms is like endurance training.

On weekdays, when dad wasn't around to help, many new mothers once rationed their liquid intake to avoid the need to go; others just stayed home. For men, meanwhile, the problem was all the more reason not to take baby out by themselves.