(Reuters) - Shares in U.S. carrier Delta Air Lines Inc DAL.N dropped 4 percent in Thursday morning trading on concerns that slowing global economies would limit the No. 2 U.S. airline's revenue growth in 2019 despite strong travel demand.

At its investor day, Atlanta-based Delta forecast a 4 to 6 percent rise in revenues, compared to an average 5.6 percent rise forecast by Refinitiv data.

The lower end of Delta’s 2019 profit forecast of $6 to $7 per share also fell short of an average $6.70 forecast by Refinitiv.

U.S. airlines hiked fares and baggage fees in 2018 to drive revenues, but investors fear carriers will not be able to keep increasing prices as global economic growth slows.

Delta shares were down $2.29, or 4 percent at $53.97 shortly before noon EST (1700 GMT), underperforming a slight rise on the S&P 500 stock index.

To boost revenues, Chief Executive Officer Ed Bastian said Delta would continue segmenting its cabins with an increased focus on higher-margin premium products. These now account for 30 percent of Delta’s revenue stream, double the amount six years ago.

The airline plans to increase average seats per aircraft by 2 percent annually between 2018 and 2023, with premium seats growing by 40 percent on every aircraft.

Delta also said it will start offering five different cabins on four of its aircraft next week, with a view to equipping all of its widebodies with the five cabins by 2021.

Less than half of revenues currently come from its main cabin.

On fuel costs, Delta said it expects a $300 million drop in fuel expenses for 2019. Traditionally, airlines have taken advantage of lower fuel costs to add more flight and seat capacity, a measure that can also drive down unit revenues, a closely watched metric that compares airlines’ sales to available seat miles.

Delta said it planned to increase capacity by about 3 percent in 2019. This is below a 4 to 6 percent capacity increase targeted by rival United Airlines UAL.O, which has been trying to claw back market share against its larger peers.

Oil prices have fallen about 30 percent since hitting a four-year high in October.

Delta expects $3 to $4 billion in free cash flow, and about $4.5 billion in capital expenditures for 2019.