"There's No Tougher Job Than Being a Mom"

This article in The Atlantic traces back phrasing of this sort to about 60 years ago, pointing out an early example from C.S. Lewis. who wrote the following in a letter to a stay-at-home mom:

I think I can understand that feeling about a housewife's work being like that of Sisyphus (who was the stone rolling gentleman). But it is surely in reality the most important work in the world.

The article also provides a bit more commentary on the distinction between 'the most important' and the 'toughest' or 'most difficult' which is worth thinking about. It seems a much better approach than the dubious suggested salary calculations that semi-regularly seem to pop up.