As she did during her first pregnancy with 14-month-old Prince George, Kate, 32, is battling hyperemesis gravidarum (HG), a severe form of pregnancy sickness that strikes between 1 and 3 percent of expectant women but is likely underdiagnosed.
The illness has already forced the princess to cancel her first planned solo tour abroad to the Mediterranean island nation of Malta, with her husband, Prince William, taking her place.
Kate is thought to be convalescing at Kensington Palace in London, where she is "able to be up and about but needs to be resting frequently," a palace source tells PEOPLE. William told well-wishers in Malta on Saturday that his ailing wife was feeling so-so.
But HG is always a brutal battle. Characterized by acute nausea and violent vomiting that leads to dehydration and weight loss, the condition is "very debilitating and very hard emotionally," says Dr. Aimée Brecht-Doscher, an OB/GYN at the Mandalay Bay Women and Children’s Group in Ventura, California, and an adviser with the HER Foundation for HG education and research.
Knowing that she had suffered from HG before – the recurrence rate is 80 percent – Kate's doctors likely intervened early. Among the actions possibly taken by her medical team, says Dr. Brecht-Doscher, are "giving [anti-nausea] medications rather than waiting until the symptoms were really bad, cutting back on activities, trying to get as much rest as possible and administering IV fluids if needed before severe nausea sets in."
When Will She Get Better?Although Kate recovered from her previous bout with HG in her second trimester, there "really is no pattern" to predict when she'll bounce back this time, says Dr. Marlena Fejzo, associate researcher at UCLA's David Geffen School of Medicine and an adviser to HER.
For that reason, when it comes to the princess's upcoming schedule, "We're taking it day by day," the palace source says.
"Generally, most women with HG feel better around 20 weeks, but 22 percent have symptoms all the way through," says Dr. Fejzo, who lost a baby in the second trimester due to HG.
In grave cases, feeding tubes are required when malnutrition is a threat.
"Women have had rib fractures, blown ear drums, esophageal tears and finger nails falling off," says Dr. Fejzo. In rare instances, the illness can be fatal to both the mom and baby.
"It's a form of starvation in pregnancy," says Dr. Fejzo, adding that many women are unable to keep even a teaspoonful of water down at a time.
"I remember sitting there and just watching the minutes go by on the clock, and that was about all I could handle," says Dr. Brecht-Doscher, who also lost a baby in the second trimester amid a battle with HG.
Long-Term EffectsFor many women, the severity of the illness has lasting implications even after the baby is born.
"About 18 percent of women have post-traumatic stress symptoms," says Dr. Fejzo, adding the experience can be so grueling that it affects their decision to have more children. Research has also shown an increased risk of medical problems for adult children of mothers who battled HG, including diabetes and heart disease.
For Kate, one of the biggest challenges is likely struggling with feelings of guilt over her reduced ability to interact with busy toddler George.
"Even the smell of your child or quick movements can make you feel extremely sick," Dr. Fejz says.
While battling HG, "the most important things are to stay hydrated and stay nourished," Dr. Brecht-Doscher says. "Emotionally, it's important to recognize that it will end, and to just try to get through every day, every hour, every minute."
With reporting by SIMON PERRY
For more on Kate's rough pregnancy, pick up this week's issue of PEOPLE, on newsstands Friday