Prince William sprung into action and drove his wife along with their personal security team 50 miles in their Range River to the hospital where Kate was placed on an IV drip.
The royal family was only notified of Kate's pregnancy mere hours before the rest of the world.
"They quickly told the queen, Prince Charles, Prince Harry and then they made the announcement at 4 o'clock yesterday," Seward told "GMA."
The announcement follows relentless public and media speculation about when Prince William and his wife would have an heir. The guessing game began almost immediately after the couple said, "I do" in April 2011.
It's customary for the royal couple to have a child within the first year of marriage. Princess Diana gave birth to William just 11 months after her wedding, and the queen gave birth to Prince Charles six days before her first wedding anniversary.
Click here for an interactive look at William and Kate's love story.
Baby rumors went into overdrive after the Duchess of Cambridge toasted with a cup of water in September.
"Well, I don't know that she would have even known she was pregnant then. But it's possible, quite possible," Seward said.
Due to a dramatic change in the rules of succession, the royal couple's first-born will likely be the heir to the throne, regardless of the baby's gender.
Last year, the heads of 16 Commonwealth countries agreed to a change in the rules of succession so that first-born children of either gender can take the throne. Queen Elizabeth II was only eligible to be monarch because her father had no male children. The British Parliament must still amend existing law to make the succession change official.
Click here for more on royal heirs around the world.
Many are speculating that Kate could be pregnant with twins because she is stricken with the condition hyperemesis gravidarum, which experts say is sometimes associated with having twins.
What does that mean for the hierarchy and the future of the throne?
"Well it would really be a case of which child appeared first," Seward said. "Whether it's male or female, the firstborn will be in line to the throne. It's that simple."
ABC News' Sydney Lupkin contributed to this report.