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Prince William and Kate Middleton marked another first in their nine-month marriage by stepping out Tuesday night in London for an official double date with William's father, Prince Charles, and his wife Camilla Parker-Bowles.

While the occasion was a rock concert at Royal Albert Hall to benefit the charities of Charles, William and younger brother, Harry, all eyes, as usual, were on Middleton, specifically whether she may be expecting the couple's first child.

The 29-year-old Duchess of Cambridge braved the cold and windy London night in a short, $99 lace tulip dress by mass retailer Zara, along with black tights, suede pumps and a blazer by Ralph Lauren.

PHOTOS: The Best of Kate Middleton's Royal Style.

"It was slightly racy for Kate," said ABC News' royal correspondent Katie Nicholl.  "At points she did look like she was trying to push the hemline down."

When not adjusting her dress, Middleton held a black clutch in front of her stomach at all times while posing for pictures with William, Prince Charles and Camilla, setting off a renewed round of speculation that she's with child.

"It's going to happen. We all know it's inevitable," Nicholl said of the likelihood England will soon welcome another generation royal.

The pregnancy rumors surrounding Kate and William first began in November when Middleton politely passed on sampling peanut paste during the couple's visit to a Denmark UNICEF distribution center.

Pregnant women are often advised to avoid peanut products to prevent allergies in their babies-to-be.  The palace did nothing to squash the immediate pregnancy speculation then when they confirmed to the press that Middleton is not allergic to peanuts.

Later in November, U.S. celebrity magazine In Touch, citing a "palace insider," reported Middleton was six weeks pregnant.

In that case, the palace was quick to deny the claims.

"We would be the ones to make the announcement, not Hollywood," the couple's spokesman told ABC News then.  "This is all complete speculation."

Despite the palace's denial, the rumor mill was further fanned by the pair's decision to trade their modest two-bedroom London apartment for more opulent accommodations, and action taken by the British parliament to change the country succession rules to  allow females to more readily become the head of the monarchy.  Any potential female child of the couple could then be queen.

British Prime Minister David Cameron was said to have initiated the change in the rules so that the first child of Prince William and Middleton could benefit.

"I think it is going to be any day or week now," Nicholl said, of when a royal announcement may come.  "And, yes, I think until it is announced we are absolutely going to be obsessed with Kate's tummy."

Tuesday night's concert was held to raise money for the areas in London hardest hit by violence and looting during the August riots.   The headline act was British pop star Gary Barlow, lead singer for the English band Take That and head judge on British version of "The X Factor."