Imports at the nation's major container ports are projected to grow 4.9 percent during the first half of 2018 compared to the same period a year earlier, as import growth slows from a torrid 2017, according to the monthly "Global Port Tracker" report published Friday by the National Retail Federation (NRF) and consultancy Hackett Associates.

The 11 U.S. ports covered by Global Port Tracker handled 1.72 million twenty-foot equivalent units (TEU) in December, the latest month for which after-the-fact numbers are available. As expected, the number was down 2.1 percent from November, since most holiday merchandise was already in the country by then, the report said. However, the December volume was up 8.4 percent year over year, and helped push the total for 2017 to 20.5 million TEU, topping 2016's record 19.1 million TEU by 7.6 percent, the report said.

January is forecast at 1.77 million TEU, up 4.1 percent from January 2017; February at 1.67 million TEU, up 14.8 percent from last year; March at 1.54 million TEU, down 1.1 percent; April at 1.71 million TEU, up 4.8 percent; May at 1.8 million TEU, up 2.8 percent; and June also at 1.8 million TEU, up 4.9 percent. The February and March percentages are skewed because many Asian factories close for Lunar New Year, which begins this year on Feb. 16.

Altogether, those numbers would bring the first half of 2018 to a total of 10.3 million TEU, an increase of 4.9 percent over the first half of 2017, the report found.

The healthy forecast for early 2018 imports reinforces the finding of a study released Thursday, in which NRF predicted that 2018 retail sales will grow between 3.8 and 4.4 percent over 2017's $3.53 trillion. NRF cautioned that cargo volume does not correlate directly with sales revenue, but said sales figures can provide a general barometer of retailers' expectations.

The predicted growth of container imports will come as consumer demand rises in response to economic indicators like high employment, improved confidence, and the recently passed tax reform law, the report found. However, those factors will not be able to replicate the hot level of growth seen in 2017, NRF and Hackett predict.

"It's clear that 2017 turned out to be a remarkable year in terms of import container volume," Hackett Associates founder Ben Hackett said in a statement. "That level of growth is difficult to sustain, however, and our models suggest that 2018 will continue to expand, but only at about half that pace, despite strong fundamentals that indicate a healthy economy and continued growth in consumer spending."