As Brian Z points out,
the whole concept of "stay at home mother" is a very modern one.
Through most of US history, it was taken for granted that (at least
for white women) motherhood was a full time occupation.
To which I would add, not only in the United States, but in most, if not all, the world - certainly in Latin America and Europe at the very least.
But not only that; the very idea that "home" and "workplace" are different locations is quite modern. The mediaeval serfs worked at "home" - women inside the house, men in the adjacent fields, except for the days dedicated to labour for the lord or the Church - when women would work in the lord's kitchen, and men in the lord's fields. The idea of a "workplace" distinct from residence, which must be frequented every day except Sundays, is a product of the Industrial Revolution, at least as a mass phenomenon.
So we really do have two centuries in which the location of labour was starkly dictated by sex: in "normal" situations, women were supposed to work "at home", as always before, and men were supposed to work in a different place - factory, office, shop - during most of the day. In situations of labour power scarcity, women - particularly poor women - worked just like as men. This is clearly visible in the begginings of industrialisation, as portrayed by Dickens or Marx, or in the reports of British factory inspectors, and was also very clearly during the two "world wars".
Even in "normal" situations, women did work outside of their homes, but their labour was usually related to "home-like" tasks - either as domestic servants or as teachers or nurses. And, of course, nuns. Related to which, it would be interesting to study in more depth to what extent and in which ways marriage and job were or were not mutually exclusive. Single women probably had a different relation to domestic tasks than married women, and celibacy as a life-long option was always open to women. Even so, there always were "stay-at-home daughters" - single women who took care of their aging parents, and helped sisters and sisters-in-law with nephews, grandchildren, disabled adults, etc.
So I would say that, at least up to WWII, rather than a gradual movement towards women employment, we had more of a see-saw movement, women extensively engaging in industry and commerce when masculine labour was insufficient (wars evidently being one of such occasions) but returning to the household as soon as conditions were less traumatic.
It is possible that this dynamics have been broken since WWII. After WWI, it took but a decade for women to move back from factories and shops into the household. And although a similar movement is easily identifiable in the 50's of the 20th century, it was never as complete as the backlash of the 30's. And then in the sixties this movement was reversed; the following depressive quarter century did not see the "normal" redomestication of women that used to be associated with economic recessions.
This coincides with the final stage of "demographic transition": lower natality rates, made possible by modern contraceptives and social security nets, make motherhood a less overwhelming job. Plus the industry has spotted an important market in the automation of domestic tasks; washing machines, vaccuum cleaners, etc., also had a significative role in freeing the female labour power from the household and into capitalist exploitation in part- and fulltime jobs.
I am not sure, however, that we are seeing an epochal change in the sexual division of labour, instead of just a particularly prolonged sway of the pendulum in the direction of employment of women. It is pretty clear to me that political demands of redomestication of women are starting to become more frequent and systematic - including this curious phenomenon of women who make a carreer out of telling other women that they should not have a carreer.
