The findings suggest that the maximum 12 weeks of maternity leave given to American mothers under federal law may be inadequate, according to the University of Maryland researchers.
"In the United States, most working women are back to work soon after giving birth, with the majority not taking more than three months of leave," study leader Dr. Rada Dagher said in a university news release.
"But our study showed that women who return to work sooner than six months after childbirth have an increased risk of postpartum depressive symptoms," added Dagher, an assistant professor of health services administration at the School of Public Health.
In the year after giving birth, about 13 percent of mothers experience postpartum depression, which can cause serious symptoms similar to clinical depression.
This study included more than 800 women in Minnesota who were followed for a year after they gave birth. About 7 percent of the mothers went back to work within six weeks, 46 percent by 12 weeks, and 87 percent by six months.
Women who were still on maternity leave at each of those time points had lower postpartum depression scores than those who had returned to work, according to the findings published online Dec. 4 in the Journal of Health Politics, Policy and Law.
The researchers concluded that "the current leave duration provided by the Family and Medical Leave Act, 12 weeks, may not be sufficient for mothers at risk for or experiencing postpartum depression."
Future discussions about maternity leave policy should take into consideration the health of mothers after they give birth, the study authors added.
They also noted that many women are not covered by the Family and Medical Leave Act or cannot afford to take unpaid leave and have to return to work sooner that what may ideal for their health after giving birth.
Although the study found an association between longer maternity leave and less likelihood of postpartum depression in women, it did not prove a cause-and-effect relationship.
The American Academy of Pediatrics has more about working mothers.