The Duchess of Cambridge, 31, told crowds in Grimsby that the baby, due in July, had been moving about "very much".
Her baby bump was visible beneath her coat as she toured the seaport on the River Humber estuary on England's east coast.
Bobbie Brown, a 42-year-old shop worker, waited for more than three hours to see Kate, the Daily Telegraph newspaper reported.
'We had a bit of a laugh'
He said: "I asked her if the baby had been moving or kicking and she said 'Yes it is, very much so'."
The baby will be third in line to the throne, behind William and his father Prince Charles, the heir to the throne.
Flag-waving crowds chanted "We want Kate" as they waited for the duchess, whose helicopter flight was delayed by the fog in Grimsby, once the world's busiest fishing port.
Former trawlerman John Vincent showed her around the Lincolnshire town's National Fishing Heritage Centre.
"She was very knowledgeable and asked some very good questions. We had a bit of a laugh going round too," he said.
"I wished her all the best with her pregnancy."
In the streets, well-wishers presented her with flowers and a Teddy bear.
Claire Moss-Smith, 86, said she had told Kate: "I'm waiting for you to be queen."
The duchess laughed, she said, and replied: "You might be waiting a long time."
Kate has undertaken a string of solo engagements since marrying William in April 2011.
The prince is expected to reveal later this month whether he will continue with his job as a military search-and-rescue helicopter pilot or devote himself more fully to royal duties.
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