Prince William and Catherine announced the news yesterday (December 3) after the Duchess was admitted to hospital for severe morning sickness. She is thought to be under 12 weeks pregnant.
Speaking to ITV's This Morning today (December 4), Dickie Arbiter commented that Clarence House and St James's Palace did "the absolute right thing" by releasing the news ahead of schedule, as "their hands were tied" by Catherine's ill health.
Acknowledging that Kate's situation is "very difficult", he said: "I think Clarence House and St James' Palace have done the absolute right thing.
"Their hands were tied and it's better to have announced something rather than rumour and speculation coming out having seen Kate walking into hospital."
Asked by hosts Phillip Schofield and Holly Willoughby if the royal couple's baby news had gone "the way they wanted", Arbiter replied: "No. They want to manage it. And certainly William wanted to manage it properly and we're looking at the very early stages [of pregnancy] probably nine or ten weeks and they would've wanted to do it properly.
"Probably [it would have been announced] after the Queen's Christmas message, because nothing really interferes with the Queen's Christmas message - so we'd be talking Boxing Day or somewhere between Christmas and New Year."
The Prince has been spotted visiting his wife of 19 months in a central London hospital today.
The couple's child will be third in line to the throne, and is also in direct succession to the crown.
A year of the Duchess of Cambridge: