It’s official: Britain’s Prince William and his wife, Duchess of Cambridge Kate Middleton, are expecting a second baby. But not all is going smooth sailing. READ: Kate Middleton Pregnancy Questions...Answered “As with her first pregnancy, The Duchess is suffering from Hyperemesis Gravidarum,” says the royal announcement revealed Monday. “Her Royal Highness will no longer accompany The Duke of Cambridge on their planned engagement in Oxford today. The Duchess of Cambridge is being treated by doctors at Kensington Palace.” When the 32-year-old was pregnant with her first born, now 1-year-old Prince George, she was hospitalized with the acute morning sickness, prompting the couple to share the news with the world. But what is the condition known as hyperemesis gravidarum and how will it impact Middleton’s pregnancy? Unlike standard morning sickness, a common side effect felt by many expecting mothers, hyperemesis gravidarum is a rare, but more severe case. READ: Is Bee Venom the Secret to Kate Middleton's Flawless Face? “Nausea and vomiting are so severe that the women can usually not even keep down fluids, such as sips of water or Gatorade,” said California-based gynecologist Elizabeth Lyster to us back when Middleton was pregnant with Prince George in late 2012. “In severe cases, it can lead to dehydration and electrolyte imbalances that require hospitalization, but, usually, resolves with conservative care and the administration of intravenous fluids,” added New York City-based gynecologist Bindi Roy-Sadarangani. According to the American Pregnancy Association, up to 80% pregnant women experience some type of morning sickness, whereas “recent studies show that at least 60,000 cases of extreme morning sickness called hyperemesis gravidarum are reported by those who are treated in a hospital.” The national health organization is also reporting that the symptoms of HG, including severe nausea, usually appear between 4-6 weeks of pregnancy and “may peak between 9-13 weeks.” While there is no known prevention, most women receive some relief between weeks 14-20. READ: Prince William Knights Kate Middleton's Ob-Gyn The condition is reportedly more common among women who are carrying more than one baby because of higher levels of pregnancy hormones. However, Roy-Sadarangani explained that "while there are more chances of HG in multiple pregnancies…this is not always associated with twins." Middleton gave birth to her firstborn in July 2013. And as expected “The Queen and members of both families are delighted with the news” of Prince George’s new baby brother or sister. The baby will also become fourth in line to the throne after its grandfather, Prince Charles, father Prince William, and brother Prince George.