I don't think it's quite accurate. Historically, lower-class mothers and a fair chunk of middle class mothers also did some work outside the home: as maids, nannies, teachers, gardeners, etc.

The 1950s USA "golden era" where lower-class mothers could afford to stay home was a statistical anomaly, gifted to this country by virtue of our unique position as the major economic superpower untouched by WWII.