The announcement of the royal pregnancy was hailed as a "new beginning" for the monarchy.
Ingrid Seward, editor-in-chief of 'Majesty' magazine, said William and Kate's child would symbolise the future of The Firm.
"It's very important for the monarchy. This is the way forward," Ms Seward said.
"This is the first child who will be an heir to the throne whatever sex they are. It's a new beginning."
She added: "I think it's wonderful. They must be really, really happy about this. It's the culmination of a terrific year and what Kate really wanted – to get pregnant in a Jubilee year."
She added that the announcement put an end to the speculation over when the couple would have a baby.
Ms Seward said the pregnancy would be a boost for the nation.
"People love weddings and they love babies. It's a happy event."
In October last year, UK Prime Minister David Cameron announced that the 16 Commonwealth countries where the queen is head of state had agreed to give female royals the same rights of succession as their brothers.
"Put simply, if Kate and William were to have a little girl, that girl would one day be our queen," Mr Cameron said at the time.
Under the ancient rules of male primogeniture, first-born royal daughters in direct line to the throne were leapfrogged by their younger male siblings. The news was also seen as the perfect way to round off the Queen Elizabeth's Diamond Jubilee year.
Having celebrated her historic 60 years on the throne, the fact that she now has a new great-grandchild on the way will ensure 2012 is even more poignant for her.
The news echoes back to the Silver Jubilee of 1977, when the queen learned that her first grandchild was on its way.
The Princess Royal's pregnancy was described at the time by the Palace as the best Jubilee present of all and Peter Phillips was born in the November of 1977. The hope and joy generated by William and Kate's baby and the Diamond Jubilee, coupled with the royal wedding in 2011, signifies a markedly different period for the royal family compared with two decades ago.
Kate's news and the Jubilee come 20 years on from the queen's "annus horribilis" year of 1992 when the Prince and Princess of Wales were at war, the Duke and Duchess of York had separated and Windsor Castle went up in flames.