Joe Little, managing editor of Majesty magazine, said the news bookended a year that saw the royal family riding high in popular esteem after celebrations of Queen Elizabeth II's 60 years on the throne.
"We're riding on a royal high at the moment at the end of the diamond jubilee year," he said. "People enjoyed the royal romance last year and now there's this. It's just a good news story amid all the doom and gloom."
Speculation about when William and his bride would start a family has been rife since their wedding.
William's mother — the late Princess Diana — got pregnant just four months after her wedding in 1981. Like Kate, Diana reportedly suffered from morning sickness for months and complained of constant media attention.
"The whole world is watching my stomach", Diana once said.
American tabloid speculation of the pregnancy has been rampant for months. One newspaper even cited anonymous sources talking about Kate's hormone levels. Others have focused on the first signs of the royal bump.
News of the pregnancy drew congratulations from across the U.K. establishment.