Chiara Ferragni's Amazon Fashion campaign.

U.S. shoppers expect to increase their online clothing purchases and Amazon is driving and benefiting from the shift.

Morgan Stanley analyst Brian Nowak noted that 52 percent of shoppers have bought clothes online in the last 12 months and this is expected to rise, with 29 percent of online clothing shoppers saying they would increase the amount they spend over the next year.

Amazon is capturing more and more of these dollars. Forty-five percent of U.S. consumers bought clothes on Amazon in the past 12 months, up from 39 percent in the first quarter of 2015. “This is the largest year-over-year increase of the 10 retailers we monitor,” Nowak said. “Amazon now has a two times-plus clothing shopper reacher advantage.”

Nowak believes that by 2018, Amazon will own 10.1 percent of all U.S. apparel spending, up from the 5.1 percent in 2015. He has also said apparel makes up 16 percent of Amazon’s gross merchandise volume.

While women still like to shop in physical stores because of fit issues, there are categories that are performing well on Amazon. Casual tops and bottoms account for 44 percent of shoppers; 36 percent are going for partywear tops, and 27 percent are shopping for dresses.

Nowak said eBay was the only retailer in the survey where the U.S. online clothing shopper penetration declined year-over-year, falling to 14 percent from 16 percent.

Many department stores continue to see their own online business grow, but now their key vendors are joining forces with Amazon in order to not miss out on this juggernaut of fashion sales. The customer can choose to buy a Michael Kors handbag on either Amazon or on Macy’s online store. The difference is in the online experience. Macy’s may also find that Amazon could offer attractive wholesale arrangements for the vendors with traditional markdown money.

Amazon is also driving this shift to online clothing purchases with its growing Prime membership. That base is up 53 percent over the past two years. Nowak said, “As Amazon Prime membership/penetration grows, so too should Amazon’s overall share of U.S. clothing and apparel spend.”