Studies of European countries with substantially longer paid maternity leave find that the benefits of up to 40 weeks off are huge. Not only does it reduce infant deaths, according to Sharon Lerner, writing for Slate:
One study tracked Norwegian children who were born after 1977, when that country increased its paid leave from zero to four months and its unpaid leave from three to 12 months, and found that the kids born after the change had lower high school dropout rates. Military draft data, moreover, tied lengthened leaves to increases in male IQ (and height, too).
Lerner looks at several studies showing benefits of longer maternal leave periods, although the reasons aren't entirely clear. It's possible that it's simply that women with better, better-paying jobs have plenty of other resources to keep their babies and children healthy and engaged.