Kate Middleton reportedly is “thrilled” to be expecting another child, but that doesn't mean her second pregnancy will come without challenges. Clarence House confirmed Monday the Duchess of Cambridge is not only pregnant with her and Prince William’s second royal baby, but she also is suffering from hyperemesis gravidarum, a pregnancy-related illness also known as acute morning sickness.

“As with her first pregnancy, The duchess is suffering from hyperemesis gravidarum. Her Royal Highness will no longer accompany the Duke of Cambridge on their planned engagement in Oxford today,” the palace said in a statement. “The Duchess of Cambridge is being treated by doctors at Kensington Palace.”

So just what is hyperemesis gravidarum and how serious is Middleton’s health scare? While some expectant mothers are prone to suffering from nausea, vomiting and weight loss, especially in their first trimester, those suffering from hyperemesis gravidarum are more at risk of experiencing severe versions of those symptoms. Some mild cases can reportedly be treated with a dietary change and rest, while more severe cases require hospitalization so nutrition and fluids can be administered intravenously, the American Pregnancy Organization said.

Symptoms usually appear from the fourth to the sixth week of pregnancy and are known to peak between weeks 9 through 13 before subsiding around weeks 14-20. Middleton is rumored to be less than 12 weeks along with her second child.

Vomiting associated with severe dehydration, nausea that does not subside, and the inability to keep food down are three major signs of the health condition. Other symptoms include, but are not limited to: headaches, confusion, fainting, jaundice, low blood pressure, rapid heart rate, anxiety and depression.

Middleton also sought medical treatment during her first trimester with Prince George in December 2012. After four days of being hospitalized at King Edward VII Hospital in London, she was photographed leaving the facility alongside Prince William. The couple welcomed their first child in July 2013.