Baby apparel is the fastest-growing category among Amazon’s apparel sales, according to a new study from retail analytics firm One Click Retail.

Baby apparel sales volume increased 67% to $50 million in the third quarter, up from $30 million in the year-ago period. One Click Retail estimates online sales figures using a combination of website indexing, machine learning and proprietary software. This data is based on first-party Amazon apparel sales, which include its private-label products and the products it buys wholesale, and does not include marketplace sales.

Young men’s apparel was the second-fastest growing segment, with sales volume increasing 45% year over year, although the segment represents only 3% of Amazon’s apparel sales, the same as the year-ago period.

Of the age and gender categories, men’s clothing represents the largest share of Amazon’s apparel sales at 41%, and women’s clothing is the next, accounting for 36%.

Overall, Amazon’s apparel sales grew 32% to $1.2 billion in Q3 compared with roughly $910 million in Q3 2016, according to One Click Retail.

By product category, underwear/intimate apparel comprises the largest segment of sales at 16%, followed by shirts and tops at 14%. Sales of shirts and tops increased the most among all apparel product categories, as the sales volume of shirts/tops sold increased 94% to $165 million in Q3 2017 from $85 million in Q3 2016, according One Click Retail data.

A large volume of underwear sales makes sense for Amazon, says Nathan Rigby, vice president at One Click, as underwear is a popular category online.

“Underwear has long been a leading category in e-commerce apparel sales, as many consumers prefer to buy their intimates in the privacy of their own homes,” Ribgy writes in the report.

The big jump in sales of shirts and tops, however, is surprising, Rigby writes.

The growth is largely because of several fall fashion trends, including luxury silk shirts for men and high-neck tops, such as turtlenecks, for women, he says. Amazon increased the number of silk shirts listed for sale by 407% year over year, and sales of this product increased 807% compared with a year ago, One Click Retail finds.

Similarly, Amazon increased the number of high-neck tops listed by 184% year over year, and sales of this product increased by 147%.