Though the words “parental leave” appear regularly in press releases and news articles, most companies and employees still think and talk primarily about maternity leave: time for a new mother to recover from childbirth, breastfeed her infant, and—unwittingly—become an expert in family management.

“My husband doesn’t know what size shoe my kid wears.” “My husband doesn’t know what time the baby naps.” “My husband doesn’t have the daycare number stored in his phone.”

These were just a few of the things mothers responded when asked how they divide household responsibility in their family. I’d posed the question because I was writing a book about what moms can do to keep their careers on track during pregnancy and parenthood, and I wanted to incorporate some tactical advice. Millions of women want a partner who is an equal partner. What can they do to get one?

In any arena, be it domestic management or the acquisition of a new professional skill, the path to becoming an “expert” is simple: practice and develop institutional knowledge. As the women I interviewed had found, they became parenting “experts” quite fast—pretty much right from when they welcomed a baby into their home.

After a baby is born, nearly all of a household’s rhythms and systems are in flux: how much the parents are sleeping, how they spend their days, what they worry about, what they buy, and about a million other things. Everything changes.

So it’s no wonder that—amid all of that upheaval—new relationship dynamics start to emerge. But, because of the way parental leave works in this country, for millions of families, those new dynamics emerge while one member is largely absent: He’s already back at work.

When I was researching my book, I surveyed more than 2,000 women and asked them how much paid parental leave their spouses had. (The survey included same-sex couples and single moms, so that figures in.) Seven percent had four or more paid weeks; 65 percent had none. And in many cases when men had paid paternity leave, there was social pressure at work not to take it. One woman, who asked not to use her name in order to protect her husband’s job, shared that when her husband applied for parental leave he was ribbed by his colleagues. She was astonished. “It’s a family-friendly company,” she said. “All of these guys have kids.” It turned out that, while they were taking some time off when their children were born (one or two weeks, usually), no one was classifying it as parental leave. The cultural norm is that dads are the “providers,” marching back into the office as breadwinners, and women are the nurturers, cloistered at home while caring for an infant.

That perception is costly to women—literally. In a 2005 study, the Cornell sociologist Shelley Correll conducted a laboratory experiment in which participants rated the competency of a series of job applicants. Overall, she found that mothers rated as “less competent, less committed, less suitable for hire, promotion, and management training, and deserving of lower salaries.” Fathers, meanwhile, seemed to benefit from their status.

Women aren’t only losing out at the the office—they’re also losing out at home. Having a baby is an opportunity for a couple to reset as a family—to put into practice all their ideals about equal partnership. But the differential in family-leave policies inhibits that from the get-go.

Being a parent isn’t much different from starting a new job. At the beginning, new employees are overwhelmed: They have responsibilities to learn, an unknown context to understand. They might feel in over their heads or sometimes even not up to the job. But as time passes—at least four weeks, but often three or even six months—they figure it out. They build up their competence and confidence, and that’s due in part to exposure and practice.

When women are home on maternity leave, they’re settling into another new job: They’re developing their skills at soothing a fussy baby and becoming experts at washing a baby’s hair without getting soap in the baby’s eyes. They’re building muscle memory for dealing with burps, and they’re getting fluent in the foreign language of baby cries.

It makes sense: The mothers are around to do these things.  When it’s time to go back to work, mothers are already so much better at those tasks that they often keep doing them unilaterally, even though she and her husband are now both at work. Over time, she takes ownership of the mundane stuff that goes into caring for a family: screening babysitters or prepping for daycare, deciding what to have for dinner tonight, and arranging outings with other families. As Kay Moffett, the head of marketing and communication for a charter-school network, calls it, the “administrivia of family life.” Sure, mom will delegate tasks to dad, but sharing an equal sense of responsibility is a much higher hurdle.

Amy Morrison, a graphic designer and writer, explained, “I spent a huge amount of time learning the ropes of parenthood and the stakes often feel too high to hand the wheel to a novice. ‘You know what, I know how to cut his nails so just give the clippers to me.’” But the cumulative effect of the nail clipping and hair washing and playdate wrangling start to take a real toll.  

By contrast, a more even split early on can last for decades. When the entrepreneur Cheryl Kellond had her first baby two decades ago, she was working for a Silicon Valley start-up with no maternity-leave policy. Financially, she had no choice but to march back into the office when her baby was three weeks old, but she negotiated to work from home half the time. Her husband made a similar arrangement, and every day, they traded roles at lunchtime. Every single day was 50-50, and sometimes they wound up with two sets of groceries, because they’d both taken the initiative to go to the supermarket. “It was both of our jobs. My husband wasn’t just doing tasks to ‘help out.’”

Kellond says she wasn’t even aware of the lasting effects of those early habits until she took a sabbatical from work 20 years later. Reading on the couch, she watched her husband walk in from a long day at work with a bag of groceries and start prepping dinner. Despite the fact that she’d been free all day (had, in fact, been free for weeks), his habit of planning dinner was so ingrained that neither noticed that there was a new imbalance because of her short-term leisure.

Their arrangement was pretty unconventional, and it likely wouldn’t fly in the vast majority of corporate settings. Until it does, pay inequality is not going to go away. While mom is developing her skill set and competency in household management, dad is investing in his career.  

But things are changing. A handful of states and private sector-employers like Facebook, Netflix, Etsy, and Change.org are starting to offer equal leave to parents of any gender. Millennial dads are eager to be deeply involved from day one, and they want to be physically present to care for their newborn baby. In doing so, they’ll gain the knowledge and expertise and shared ownership of that “administrivia” that will make them truly equal partners. That physical act of being around a baby all day will have huge consequences—not just tomorrow but next month and next year and next decade too.