The baby bottoms of Americans born before the 1980s likely never touched a diaper-changing station in a public restroom. Prior to the ’80s, when parents, and mothers in particular, went to shop or go out to eat, they usually had to fold themselves into the back of a car, balance their wriggling infant on a toilet seat, or crouch on a dirty bathroom floor to change their child’s diaper.

In the decades since, changing tables have grown more common, but they still can be hard to find, especially for dads. That is slowly changing: Last fall, President Obama signed a bill that will require all bathrooms in buildings controlled by the federal government to provide baby-changing stations, including in men’s rooms.

The placement of changing tables may seem like a minor design decision, but their availability relates to shifts in the larger patterns of care and work. Over the last 100 years, the availability of changing tables has tracked remarkably closely with trends in American parenting. The history of the device—as well as its future, as hinted at by that new law—is intertwined with the increasing number of dual-income households and the popularity of products designed with parents’ convenience in mind, as well as some of the most important recent changes in how Americans spend their days.

Well before parents came to expect publicly-available changing tables, many mothers simply limited their activities away from the home, in part to avoid uncomfortable diaper changes. It was this response that inspired an early and surprising champion of changing stations: the power-hungry, domineering New York urban planner Robert Moses. Before Moses—the man who would be immortalized in Robert Caro’s 1974 book The Power Broker—oversaw the creation of a network of highways, the United Nations building, and numerous public-housing complexes, he made mothers’ lives easier. On a weekend stroll in 1914, the labor advocate and sociologist Frances Perkins told Moses that mothers have to trudge home from Central Park every time their baby’s diaper needs changing—their time in a splendid public space was being cut short. A young and visionary civil servant, Moses imagined diaper-changing stations dotting New York’s parks, enabling mothers to enjoy more uninterrupted time outdoors.

Moses’s diaper-changing rooms—the first of their kind—became available on New York’s Jones Beach starting in 1929. But it would take decades for these changing stations to become common features of public restrooms, and what eventually made the need for them more pressing was a number of shifts in family norms. A half century after the new rooms appeared on Jones Beach, more mothers held jobs and the number of single parents and dual-income couples had risen. Working mothers, short on time as they were, needed to be able to take their kids with them when they ran errands. In 1999, Nate Klatt, the marketing coordinator for Koala Corporation, which produces changing tables, summarized what was happening for the automotive trade magazine Ward’s Dealer Business: “High divorce rates, working parents’ guilt over not spending enough time with their kids and high-priced child care have conspired to make children active sidekicks in the parents’ social lives.”

These sidekicks started joining their parents at malls, restaurants, and airports, but such spaces weren’t equipped for them. As parents—particularly mothers—spent more time outside the home, they needed a place to change their baby, who was more likely to be perched in a small stroller than stretched out in a pram that was large enough to allow for discreet diaper-changing.

This was the very problem Jeff Hilger, a medical-device salesman and father living near Minneapolis in the mid-’80s, found himself facing. He and some friends came up with the idea of a fold-out device that could be mounted to a wall, letting parents take care of messy business while on the go. They called it the Koala Bear Kare Baby Changing Station, and sales took off as businesses came to recognize the value in accommodating children. As an L.A. Times reporter noted in 1990, “This is the new world of retailing, where the diaper bag meets the shopping bag.” Realizing that convenience for parents would let them spend more time, and thus money, businesses like McDonald’s and Target soon snapped up wall-mounted changing stations.

These changes spawned a number of other products that catered to on-the-move parents who had kids in tow. The breast pump came onto the market at around the same time, as did more-advanced car seats. Gone were the stately and spacious prams that once transported babies around the neighborhood; instead, parents started wheeling their kids around in compact strollers or strapped their babies into a Snugli, a carrier that sits on parents’ chests and lets them carry their kids around as if in a kangaroo pouch. With a Snugli, parents could keep their infant physically close, even while walking around or running errands.

These innovations reflect a change in how parents spent time with their children, according to Paula Fass, a professor of history at the University of California, Berkeley and the author of The End of American Childhood. In the 1980s, parents were moving in two directions at once: They spent more time on leisure activities, but they also sought to maximize the time they spent with their kids. Parents, Fass says, wanted “the child to be able to fit comfortably into their new lifestyle. Instead of adapting to the child by staying at home, you have the child become part of the pattern.” Devices like the wall-mounted changing table and chest-strap infant carriers made on-the-go caregiving convenient.

This represented a departure from the way mothers used to plan their days. Fass says that mothers used to “literally [stay] at home with their children.” While they cooked or cleaned, mothers would put their kids in the playpen or let them play outdoors. In the 1980s, “Women not only went out to work, but they engaged in a whole lot of other activities that took them outside of the home, to which they felt comfortable—increasingly comfortable—bringing their children. And that’s true with fathers too.” In other words, there came to be more and more spaces where parents spent time with their children.

But even as diaper-changing stations have done a lot to help mothers operate more flexibly outside the home, they haven’t done much to alter the traditional division of child-rearing labor. A changing table is much likelier to be found in a women’s restroom than a men’s restroom. This imbalance keeps fathers from taking care of their children while out and about, and poses an everyday challenge to men in same-sex partnerships.

Changing tables’ absence from men’s rooms harkens to an earlier time, when fathers were usually responsible for fewer of the everyday tasks of parenting. Yet mothers and fathers have slowly moved toward parity in housework and childcare, and some surveys have suggested that young Americans increasingly expect parenthood to be an equally-divided endeavor. As one New York state senator who has advocated for equal access to changing tables wrote in 2015, “If we expect fathers to bear more of the burden of child-care, we must ensure that public accommodations reflect this new normal.”

This gender disparity has led local and state politicians to propose legislation around changing tables, in places like New York state, Honolulu, and California. In 2015, the Californian actor and investor Ashton Kutcher drew attention to the absence of changing tables, lamenting in a Facebook post, “There are NEVER diaper changing stations in mens [sic] public restrooms.” He drafted a Change.org petition calling for Target and Costco to install changing tables in men’s restrooms. It garnered over 100,000 signatures.

The new federal law that Obama signed, the Bathrooms Accessible in Every Situation Act, follows this momentum, requiring changing tables in men’s restrooms. But the law only applies to federal buildings, like post offices and courthouses, which comprise just a tiny fraction of public restrooms. As the social landscape continues to shift, the physical landscape still has catching up to do.