She was forced to cancel a scheduled engagement today because of her health, Kensington Palace said in a statement announcing her pregnancy.
Dr. Jennifer Ashton, a practicing OB-GYN and ABC News' chief women's health correspondent, described the condition as "very, very serious."
"We're talking about the potential for severe malnutrition, electrolyte abnormalities, sodium, potassium [abnormalities] and in some cases organ failure," Ashton said.
Kensington Palace announced her first pregnancy shortly before she was admitted to King Edward VII Hospital in London for treatment.
With this third pregnancy, Kate is being treated by doctors at Kensington Palace, as she was when she was carrying Charlotte.
Nausea and vomiting in pregnancy are believed to be caused by a "rapidly rising blood level of a hormone called human chorionic gonadotropin," according to the NIH.
Hyperemesis gravidarum's severity can vary, from some women being "bed bound to being hospitalized," according to Ashton.
Treatment for hyperemesis gravidarum depends on the severity of symptoms and the treatment's effects, Ashton said.
"Treatment really at some points can need IV hydration," she said. "Typically we use some medications that work centrally in the brain and in the [gastrointestinal] tract," she said.
"And then we're talking about potentially some rest, some certain diets, some alternative or complementary therapies, and again, I have seen pregnant women need IV or even tube feeds for nutrition and hydration."
Kensington Palace has not announced how far along Kate is in her pregnancy. If she is at the same stage of this pregnancy as she was when her previous pregnancies were announced, the baby may be due around April or May.
George was born on July 22, 2013, and Charlotte was born on May 2, 2015.
Ashton noted the impact hyperemesis gravidarum may have on entire families, saying it is important for both the pregnant woman and her family members to "take it day by day."
Women who have experienced hyperemesis gravidarum are at higher risk for the condition to recur, according to the NIH.
William and Kate's expected third child will be fifth in line to the British throne.