But Kate, who is five months pregnant, said that the duke, who attended the event with her, would prefer to have a girl.
Ahead of watching the parade at Mons Barracks in Aldershot, Hampshire, the duchess suffered an embarrassing mishap when the heel of one of her shoes became stuck in a drain.
Kate, who showed patriotic spirit in the same green Emilia Wickstead dress coat she wore to the event last year, had to lean on William while she pulled it out with her hand.
Afterwards, the couple chatted to soldiers from the 1st Battalion Irish Guards in the guardsmens’ cookhouse.
Guardsman Lee Wheeler, 29, said: “I was talking to her about the baby, of course. I asked her ‘do you know if it’s a girl or boy,’ and she said ‘not yet.’
“She said ‘I’d like to have a boy and William would like a girl.’ That’s always the way.
“I asked her if she had any names yet and she said no. I said I suppose you’ve got to stick to traditional names.”
Guardsman Jason Perry, 33, also spoke to Kate about her pregnancy.
“I wished her congratulations and said I hoped everything is fine.
She said ‘yes it is.’ I asked her if she was excited to be having her first child. She said ‘very,’” he said.
The soldier said it was “absolutely brilliant” to meet the couple and they had both asked him about his 13-and-a-half year career in the army, which has seen him serve two tours of Iraq and one in Northern Ireland.
William, who wore the ceremonial dress of the Irish Guards, sipped on a glass of sherry inside the dining hall while Kate went without a drink.
Before going inside, the royal couple had put on a brave face as they posed for formal photographs with officers and sergeants in the torrential rain.
The wet weather had earlier held off as the royal couple watched the parade take place under the cover of a podium.
Under grey skies, the duchess took her position next to the duke, who was attending the event as colonel of the regiment and for the first time wore an insignia aide de camp to the Queen on his shoulder, which was bestowed on him yesterday.
The couple looked on as about 200 soldiers paraded through the square, led by the band of the Irish Guards.
The duke watched as Kate presented sprigs of shamrock to the officers and guardsmen, including one to himself.