These days, when cruise lines say they want to baby their passengers, they’re not necessarily talking spa treatments and butler service.

They mean rental strollers and onboard nurseries.

Yes, it’s cruising for the diapered set. And by many accounts, these tiny passengers are becoming a bigger part of the $37 billion cruise industry, one that gives new meaning to the term, “Baby on board.”

Just this week, MSC Cruises, an Italian-based cruise line and Chicco (pronounced kee-ko), an Italian-based baby care brand, announced a partnership in which Chicco will design “innovative” infant and toddler-oriented areas aboard MSC ships. Additionally, Chicco will provide moms and dads with all the requisite baby gear, from bottle warmers to baby backpacks, while on board. “It allows parents not to have to pack as much,” says Ken Muskat, an MSC executive vice president.

Other cruise lines are also hopping aboard the baby bandwagon. This summer, Norwegian Cruise Line NCLH, -5.85% is running a “Kids Sail Free” promotion though Aug. 2. And in the past few years, Royal Caribbean International RCL, -4.80% has been expanding its Royal Babies and Tots Nursery program across many of the ships in its fleet. (Royal Caribbean also boasts a “Babies 2 Go” program where parents can order diapers and organic baby food, among other items, for delivery to their stateroom upon arrival.)

The ins and outs of cruise-ship tipping

And that’s not to mention the cruise line that’s largely responsible for jump-starting the trend—namely Disney DIS, -1.22% , which launched its kid-friendly cruise line nearly two decades ago. Interestingly, Disney just announced that it’s changing the minimum age on its cruises—from 12 weeks to six months--on most of its itineraries. “These minimum age restrictions are consistent with cruise industry standards,” said a Disney spokesperson of the change. (Sure enough, most other cruise lines have a six-month minimum.)

But given the fact the Disney Cruise Line isn’t quite a new kid on the block, why the cruise industry’s newfound emphasis on infant travelers? Most industry observers say it’s not about infants per se. Rather, it’s about cruise lines trying to woo multigenerational groups. Apparently, the way of late to get grandpa and grandma to travel is to let them bring their grandkids on the trip. Indeed, one-third of grandparents vacationed in this very manner over the past year, according to a recent marketing study.

Also see: Pitching cruising as a multigenerational vacation.

But until relatively recently, if an infant had to be accommodated as part of a multigenerational group, it often meant the trip was a no-go, simply because cruise lines didn’t have the requisite services and amenities. That is, until the cruise lines got wise, say industry observers. With so much to be gained financially by welcoming large families, cruise lines “didn’t want bringing a baby to be seen as a deterrent,” says Colleen McDaniel, managing editor of the Cruise Critic website.

Still, just because a cruise line makes it easy for a child under three to be accommodated doesn’t mean every parent—or grandparent—may want to go this route. McDaniel says costs can be an issue, since some cruise lines charge standard prices for infants, despite the fact they won’t exactly be eating their fair share at the midnight buffet. And given the potential for virus outbreaks on cruises, some health experts warn that infants and toddlers, who have naturally weaker immune systems, might not be as well-protected.

At the very least, says Dr. Dennis Murray, a pediatric infectious disease specialist, if you bring your little ones with you on your next cruise, “make sure they’re getting their hands washed.”

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