Prime growth is plateauing in the U.S., showing the first signs of a slowdown in its oldest market, Morgan Stanley wrote in a note published this week.

Prime, a paid membership program that gives free two-day shipping and access to other perks such as online video and music, is important for Amazon because Prime members tend to spend more on its e-commerce site. Amazon calls Prime one of the three pillars of its business, alongside the Amazon Marketplace, its site connecting buyers and sellers, and Amazon Web Services, its cloud computing business.

Based on a survey of 1,000 U.S. adults in the third quarter of 2017, Morgan Stanley concluded that 40 percent of American consumers are currently Prime members — the same percentage seen in its survey from the fourth quarter of 2016.