By Kelly Wallace

CNN

(CNN) -- We've certainly come a long way in the paid leave debate. For the first time, Democrats and Republicans believe there are enough benefits to paid leave that government should play a role.

Just how much time should be covered will be the subject of political debate in the weeks and months ahead.

The research is clear. Based on CNN's review of more than 20 studies on the health impacts of paid parental leave on parent and child, we found that most studies come to the same conclusion: Paid parental leave can have a significant positive effect on the health of children and mothers.

But how much time is enough? Forget politics for a moment, and how a paid leave system would ultimately be financed. How much time is best for babies, for mothers, for fathers?

That is exactly the question journalist and author Brigid Schulte and her colleagues at the New America Foundation tried to answer in their recently released report.

"In reporting on (paid leave) over the years, everybody looks sort of longingly at Europe. It's like, 'Oh my God, they have months. They have weeks. They have years.' Well how much is enough?" said Schulte, director of the Better Life Lab at New America.

Anyone who follows this issue knows the United States stands in an embarrassing position as the only high-income nation not to have a national paid leave policy. It is also one of only a few countries around the world without such a plan. (Those others include Papua New Guinea, Suriname and a few small South Pacific Islands.)

Schulte wondered if the science would show there was a "sweet spot" for the duration of paid leave for mothers and fathers.

After conducting a deep dive project, where Schulte and her New America colleagues reviewed numerous studies and talked to a range of experts, they have a recommendation for the optimal duration of paid leave according to available science: Based on infant health, maternal health, gender equality and female labor force participation, they recommend six months to a year of paid family leave.

The timing for such a conversation couldn't be more important, Schulte said, especially as President Donald Trump is calling for six weeks of paid parental leave. Ivanka Trump recently discussed the President's plan with Republican Sen. Marco Rubio of Florida and other Republican members of Congress. A Democratic alternative calls for 12 weeks of paid parental leave.

"If we're the last country to do something like this, we really have the opportunity to learn from what other people have done," said Schulte, author of The New York Times best-selling book "Overwhelmed: Work, Love and Play When No One Has the Time."

"The bad news is the United States is so late to the game," said Jody Heymann, dean of the UCLA Fielding School of Public Health, who has been leading teams studying paid leave for 17 years. "The good news is that with 187 countries in the world having paid maternity leave, over 90 having paid paternity leave, actually there's a ton that's already known about what works."

Heymann, who is also founding director of the WORLD Policy Analysis Center at UCLA, is leading a comprehensive review of the literature on paid leave, including the health impact of different durations. That review is expected to be released over the summer or in early fall.

Where did we get 6 weeks, 8 weeks, 12 weeks?

If you live in the United States and have given birth, you are probably familiar with these three stretches of time: six weeks, eight weeks and 12 weeks.

Six weeks is the amount of time you may be eligible for some disability pay after a vaginal delivery and eight weeks after a cesarean section. Twelve weeks is the amount of unpaid leave you may be eligible for under the 1993 Family Medical Leave Act, as long as you're at a company with 50 or more employees, and have worked at that company for at least a year, and at least 1,250 hours during that year.

What many people might not realize is that the initial Family Medical Leave proposal, which ultimately got watered down, was six months of paid leave.

"Some of the very early hearings, they had pediatricians and medical experts saying if you were to do this, a minimum should be six months paid," said Schulte.

In 1978, after the Pregnancy Discrimination Act became law, women were eligible to apply for six to eight weeks of partial disability pay after the birth of a child. The number of weeks was based on an understanding back then of what it would take for the body to heal and for the major reproductive organs to return to a nonreproductive state, said Schulte.

"What we were able to find is that there have been a number of international studies since then that have really questioned that six to eight weeks and show that a majority of women at six months are still experiencing one or more symptoms of physical illness, whether it's dizziness, fatigue or urinary incontinence or a host of things," she said.

"You are not healed in six months, whereas back in 1978, they thought you were, but again, that was based not so much on science as on what were the typical amounts that you could get under disability insurance programs."

Today, one in four US mothers returns to work much earlier -- two weeks after childbirth -- according to Schulte's report.

What the science shows

"The research is really clear, particularly when you look at infant mortality," said Schulte. Of all advanced economies, the United States has one of the highest rates of infant mortality and sudden infant deaths.

A study of 20 low-income and middle-income countries found that for each month of paid maternity leave, there was a 13% decline in infant mortality. The greatest reduction in infant mortality was found with 40 weeks paid leave, according to the report.

"From the infant health standpoint, it matters for women to be able to exclusively breastfeed for six months. The best way to guarantee that they can exclusively breastfeed for six months is to have at least six months of paid maternity leave," said Heymann of UCLA. "Why care so much about breastfeeding? Because breastfeeding lowers infant mortality three- to five-fold in high- and low-income countries."

The benefits for children can also be long-term, according to the research. Researchers compared the lives of children born in Norway before 1977, when mothers had 12 weeks of unpaid leave, with children born after, when the country offered an additional four months of paid leave. The children whose moms had longer leaves had better cognitive and academic development at age 30 and were more likely to have graduated from college and have higher wages.

When it comes to maternal health, an Australian study found that psychological distress was significantly less likely for mothers who took more than 13 weeks of paid leave.

The report also noted that leaves shorter than 12 weeks have been connected with higher depression and anxiety, a decline in self-esteem and problems with sensitivity to the baby.

"We have one of the highest infant mortality rates of developed countries. We have one of the highest maternal mortality rates of developed countries. I mean higher than Bosnia ... places that have just come from recent conflict. And when you put it all out on a time line, you really start seeing connections about the larger cost of not doing some kind of paid family leave," said Schulte.

Schulte and her colleagues did not find a study that backed a particular length of time to get the best results for gender equality, but what they did find is that when men took an adequate amount of leave, there was a huge impact on the gender division of labor.

Studies out of Iceland and Quebec showed that three years later, men who had taken paid leave were more likely to share child care and divide up other responsibilities at home.

"We found that having some kind of equal bonding would actually disrupt those powerful traditional gender roles and enable families to form their own kind of work and life combination in a way that was best for their family," said Schulte.

When it comes to women returning to the workforce after giving birth, a bell curve exists. When leaves are too short, women tend not to go back or they simply can't go back, and if they are too long, they can't easily get back in, according to the report.

For example, in the Czech Republic, where with paid leave and home leave, women could be out for three years, women tended to not go back or employers didn't want to hire them back.

"The question has been raised whether the very long leaves that some European countries have, meaning two years and over, not a length the United States is considering, do those have any impact on gender equality?" said Heymann, who said her research study is looking into that very issue.

"There's absolutely no evidence that there's any negative effect on workplace equality of the durations the United States might consider," she added.

"There's this sweet spot that most studies will show it's between nine months to a year where nine months is where it's positive and up to a year. Anything longer than a year tends to be kind of a detrimental effect to women's wages, women's return to work and their career prospects," said Schulte.

How do you pay for it?

Heymann's research will also look at how paid leave is financed around the world. Does the government pay for it? Does the employer or the employee or some combination? She and her team will then compare those results with impacts on the health of the infant and parents, equality between men and women at work and home and the economy.

"The reason we're mapping onto economic outcomes is one of the questions that is often raised is, 'Can countries afford to compete and do this?' " said Heymann. Her team is trying to answer whether there are certain models that are more or less feasible.

Schulte's report points to a 2017 survey conducted by Ernst & Young, which found that 90% of companies with paid family leave policies found there was either a positive or neutral impact on productivity, morale and the bottom line.

Studies of the three states currently offering paid leave, California, New Jersey and Rhode Island -- New York's plan goes into effect in 2018 -- found that a majority of companies experienced positive or neutral effects on productivity.

In the United Kingdom, a survey of more than 2,000 companies found that workplaces with parental leave policies were 60% more likely to report that their financial performance was higher than average as compared to the companies that didn't offer any leave plans.

"People like to talk about paid family leave as like a perk or an accommodation or kind of nice to have, but when you look at the totality of all of these studies, it's really an investment in the future," said Schulte.

Schulte hopes the report she and her colleagues crafted will help people understand, even before the debate begins, what, let's say, six weeks of paid leave, the President's proposal, means based on the science and the available evidence.

"We're not going to be Estonia where they have 166 weeks, but we'll hopefully move beyond zero. It looks like there is movement so what is the right amount of time? Where do we start?" said Schulte. "The whole goal here is to really bring some objective, as much as it can be, neutral scientific data and really inject it in the conversation and to help guide people making the best decisions."