Twelve of the companies provide paid leave to salaried new parents who did not give birth, like fathers and adoptive, foster or same-sex parents. Eight provide it to nonbirth parents who are paid hourly.

Americans are not ambivalent about whether paid leave helps workers. In a recent Pew Research Center survey, 94 percent said it would help families, and two-thirds said it would help the economy. In addition to an economic issue, paid leave has become a political one, and advocacy groups like Paid Leave for the United States and Organization United for Respect have been lobbying companies to offer it.

The need is most acute for hourly employees, who are 59 percent of the American work force. They are least likely to be able to afford unpaid leave or newborn child care, and more likely to drop out of the labor force after giving birth.

A mother’s physical recovery from childbirth generally takes at least six weeks, doctors say, and there is an emotional adjustment period as well. Newborns demand frequent feedings, on an irregular schedule, and it is hard for mothers to breast-feed or rest if they return to work immediately. Most day-care providers do not offer newborn care, or it is very expensive.

In the Pew survey, 16 percent of people employed in the last two years said they needed to take leave but were unable to. They were more likely to be low earners, and also more likely to be women, black, Hispanic or without a college degree.

Research has found that mothers who take paid parental leave are more likely to be working a year later, and less likely to receive public aid. It also improves mothers’ physical and mental health; the duration of breast-feeding; babies’ health and development; and gender equity.

When fathers take such leave, research shows, they are more involved in their children’s lives years later; their children are healthier; and mothers have increased earnings and better mental health.