Kate Middleton is "continuing to feel better" as she spends her second day at King Edward VII Hospital in London, according to St. James Palace. She may have to stay several more days to recover from severe nausea related to her pregnancy.
"The Duchess of Cambridge is continuing to feel better," a St. James Palace spokesman said. "She and the duke are immensely grateful for the good wishes they have received. She will remain in hospital at present and will continue to be treated for hyperemesis gravidarum."
William left the hospital at around 5:45 p.m. local time. He appeared relaxed and smiled at reporters before being driven away. Prince William has been the only member of the royal family seen visiting Kate in the hospital, where she is being treated for acute nausea.
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As Kate rests and recovers, royal experts foresee an "open" and "ordinary" pregnancy for a modern couple in an increasingly modern monarchy.
"I think we will see a very ordinary pregnancy, a very open pregnancy," royal historian and author of "The Queen - A Life in Brief" Robert Lacey told "Good Morning America" today. "Already the details of what's happening to Kate are not being hidden as would have been the case in the past. And we're seeing the concern of a very modern husband going to see her in [the] hospital every day."
Lacey predicts that the birth of William and Kate's baby could mark the first time the heir's father is in the hospital for the birth.
"When Charles was born, Prince Philip played squash here at the palace. When Prince William was born, Prince Charles went off and played polo," Lacey said. "I think we're getting to see the first royal birth where the father himself is present at the birth and sees his new child come into the world."
Lacey said that the royal couple also bucked royal tradition by not having a child in the first year of their marriage.
"The Queen and Diana had their first child within a year," he said. "We wondered why they were waiting and now it's clear. They didn't want to upstage the Queen's jubilee."
The duke and duchess had a number of royal obligations to fulfill this year, including in international Jubilee tour on behalf of the Queen.
Another factor that played into them waiting, Lacey said, was that their new home in London is going to be ready for them in April.
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A palace source told ABC News that the royal couple decided to go public with the pregnancy because Kate had to be hospitalized.
Kate was admitted to the hospital following the announcement that she is expecting and she may have to stay there a while longer.
The Clarence House confirmed to ABC News that Kate will be in the hospital for "several days," and will then require a "period of rest" battling severe morning sickness.
"Well they didn't want to let it out because she's still probably about the 10- week mark but they knew that once Kate was seen going into the hospital… that it was going to be out of their hands and then the media would have control of the announcement," Ingrid Seward, editor-in-chief of Majesty magazine, told "Good Morning America."
William and Kate were spending time with her parents in Bucklebury when she became ill with the symptoms of hyperemesis gravidarum, or acute nausea.
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.
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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.
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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.