As this NYTimes Magazine long (but great!) story points out, there is not one answer to this question, but the right-wing canard about uppity working women who refuse their "duty" to bear the seed is exposed as bullshit:
They found that a greater percentage of Dutch women than Italian women are in the work force but that, at the same time, the fertility rate in the Netherlands is significantly higher (1.73 compared to 1.33). In both countries, people tend to have traditional views about gender roles, but Italian society is considerably more conservative in this regard, and this seems to be a decisive difference. The hypothesis the sociologists set out to test was borne out by the data: women who do more than 75 percent of the housework and child care are less likely to want to have another child than women whose husbands or partners share the load.