The fewer babies Americans give birth to, the more small dogs they seem to buy.

Birth rates in the US have fallen from nearly 70 per 1,000 women in 2007, to under 63 last year—a 10% tumble. American women birthed almost 400,000 fewer little humans in 2013 than they did six years before. The drop-off has come exclusively among 15- to 29-year-olds. This chart, taken from a recent report by the US Department of Health (pdf), does a pretty decent job of showing how much of the growing disinterest in having babies is due to younger women:

Meanwhile, the ownership of small dogs—that is, pets weighing no more than 20 pounds (9 kilograms)—is doing just the opposite. Americans have been buying more and more small dogs each year since 1999. The population of little canines more than doubled in the US over that period, and is only projected to continue upwards, according to data from market research firm Euromonitor.

“You do not have to go to many pet shows to realize that the numbers of small and tiny dogs are on the increase,” a report by Pets International opened in 2010 (pdf).

And rightly so. The number of small dogs has grown so fast that they are now the most popular kind nationwide.

It could just be a coincidence that Americans are birthing fewer babies at the same time as they’re buying a lot more little dogs. But there’s pretty good reason to believe it isn’t, Damian Shore, an analyst at market-research firm Euromonitor, told Quartz. “There’s definitely some replacement happening there,” he said.

One telling sign that the two are not entirely unrelated is that the same age groups that are forgoing motherhood are leading the small dog charge. “Women are not only having fewer children, but are also getting married later. There are more single and unmarried women in their late 20s and early 30s, which also happens to be the demographic that buys the most small dogs,” Shore said.

There’s also evidence people are treating their dogs a bit more like little humans these days. Premium dog food, the most expensive kind, has grown by 170% over the past 15 years, and now accounts for 57% of of the overall dog food market.

There are now tools to monitor your dog’s fitness, ice cream trucks exclusively for canines, and vacations designed exclusively for dog-having people. “The animals in our homes are family. They’re like children,” David Grimm, the author of the book Citizen Canine, told Wired this week.

Of course, small dog ownership isn’t rising just because people want kid substitutes. Fashion trends aside, small dogs are also emblematic of a national migration to cities, where big dogs are harder to keep. Nearly 80% of Americans live in urban areas. ”Smaller homes and apartments are also helping drive the growing popularity of smaller dogs,” Shore said.

But the national trend towards later motherhood is certainly playing its part. And those who treat their pooches and pugs like babies may be on to something. A study last year found that dogs form bonds of dependency with their owners not unlike the ones babies form with their parents.