Moreover, their babies were less likely to die, had better odds of secure maternal attachment, and were more likely to be breastfeeding and get timely vaccinations, the researchers added.
"In the U.S., women in higher-paid households are often able to stay home with their infants for 12 weeks or more because many of them have access to paid maternity leave, or are able to take unpaid leave without significantly impacting their families," said researcher Dr. Christina Mangurian. She is from the University of California, San Francisco's department of psychiatry and the Weill Institute for Neurosciences.
"On the other hand, lower-income women frequently need to return to work earlier because they can't afford to take unpaid leave," Mangurian said in a university news release. "This is a problem because data suggest that at least 12 weeks of paid maternity leave has a beneficial effect on the mental and physical health of both the mother and baby."
For the study, Mangurian and her colleagues reviewed 26 national and international studies that examined the effect of paid maternity leave and duration of leave.
The investigators found that adding 10 weeks of paid maternity leave to the average paid leave in each of nine western European countries reduced the number of infant deaths by approximately 5%.
A U.S. study of more than 3,300 mothers found that less than eight weeks of paid maternity leave was tied to poorer health and increased depression.
Another U.S. study of more than 1,900 mothers found that those with more than 12 weeks of paid maternity leave were more likely to keep up on vaccinations, had kids with fewer behavioral problems and had breastfed for longer periods.
A third U.S. study of more than 3,800 mothers found that longer duration of paid maternity leave led to better mother-child interactions, and more secure attachment, empathy and greater school success.
Paid-leave policies in the United States often create a two-tiered system in which those with higher incomes can afford to stay at home for 12 weeks or more, while lower-income women have to go back to work, the study authors explained.
Of those who earned less than $30,000 per year, 62% didn't receive any paid leave, compared with 26% of women who made more than $75,000 per year, the findings showed.
Based on earlier studies, the researchers suggested that paid maternity leave could help the economy by "substantial individual and societal benefits, notably labor force attachment, wage stability and decreased use of public assistance."
According to study co-author Dr. Maureen Sayres Van Niel, a reproductive psychiatrist in Cambridge, Mass., "For decades, national paid maternity leave policies of 12 weeks or more have existed in every industrialized country except the United States. We recommend that the United States develop a national paid policy that would allow all mothers sufficient time to be home with their infants, regardless of their employer or socioeconomic status."
The study was published in the March/April issue of the Harvard Review of Psychiatry.
For more on paid maternity leave, head to the Pew Research Center.