I'm not sure this argument holds... Japan has falling birthrates and non-working women. Maybe the cause is somewhere else? Maybe we could legislate that many (most) jobs must take part-time workers (e.g. in Netherlands IIRC), and that parental leave is covered by the government, and that both employee and employer get a bonus when a child is born (correlated with the previous salary, so that more productive/educated people are more incentivised to have kids).