This is not the answer I would have predicted:
The impact is identified by legislated increases in the duration of maternity leave in Canada, which significantly increased the amount of maternal care children received in the second half of their first year.
…Our results indicate that these changes had no positive effect on indices of children’s cognitive and behavioral development. We uncover a small negative impact on PPVT and Who Am I? scores, which suggests the timing of the mother/child separation due to the mother’s return to work may be important.
Of course, I believe maternity leave in Canada went from “long” to “very long”. In the US, where I believe they give you the first six hours off (pay docked, of course) and where there’s a little more inequality, I might expect more impact.
Jeannie works for a major humanitarian organization who have no maternity leave policy, and had to take accumulated sick leave. She recalls a European colleague, who leaned across the desk of the human resource director: “Having a child,” she said, “is not a sickness.”