(Bloomberg) -- Kimberly-Clark Corp., owner of the Huggies brand, is changing up its advertisements after Procter & Gamble Co. challenged the brand’s claim to be the best-fitting diaper.

Huggies began touting its Little Movers line of diapers as the best-fitting in an ad campaign that debuted across the US less than a year ago. The company marketed its curved elastic and pad as better fitting baby bottoms in TV commercials, as well as on digital and social media. Pampers and Luvs owner P&G recently disputed those claims through the National Advertising Division of BBB National Programs, a nonprofit that helps industries with advertising self-regulation and accountability. 

Dallas-based Kimberly-Clark later agreed to remove or alter its advertisements, including a video in which a narrator states: “Only Huggies is the No. 1 fitting diaper, with a curved and stretchy fit.” A Kimberly-Clark spokesman confirmed the company is complying with the NAD’s ruling. P&G declined to comment.

Information that Kimberly-Clark provided on different studies used to support its best-fitting claim showed flaws, rendering it unreliable, according to the NAD’s final decision, viewed by Bloomberg News. Those flaws included using manual measurements with a ruler instead of image analysis to measure fit, along with one study that had a disproportionate gender ratio of 19 girls to 6 boys. The company also didn’t provide the raw data from the studies, or any statistical analysis of the results. Kimberly-Clark didn’t immediately respond to a request for comment on the studies.

“If we do not have sufficient information about how products are compared, we cannot conclude that the testing is sufficiently reliable,” Laura Brett, vice president of the NAD, said in an email.

The marketing conflict shows the intense rivalry between Kimberly-Clark and P&G, the two largest diaper makers, which together comprise more than half of the domestic market. A stagnating US birthrate has put pressure on these companies’ to show growth in diapers, a category that data provider Circana values at $5.9 billion a year. Unit volumes declined last year and in 2022, as Americans pushed back against persistent inflation in addition to having fewer babies.

Huggies has been trying to boost sales recently with new moisturizing baby wipes, along with its lines of fragrance-free diapers, while P&G has developed bigger sizes and overnight training pants for older children. Kimberly-Clark has also been touting certain premium features, including the fit of its diapers developed to fit a variety of baby shapes and sizes, while also selling Huggies-branded baby clothes at retailers like Target.

Cincinnati-based P&G said last week that it’s struggling to boost its Luvs business in the US, while the higher-priced Pampers Cruisers and Swaddlers have been growing. Shipment volumes fell 3% for P&G’s segment that includes baby items in the latest quarter.

“Where we have an opportunity in the US is on the midtier, on Luvs,” Chief Financial Officer Andre Schulten said during a conference call. “We have not been able to push the full innovation pipeline out for different reasons.”

The companies have been on opposite sides in past ad-related dust-ups. P&G agreed in 2017 to change some claims about its Easy Ups training pants after Kimberly-Clark complained to the NAD.

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