While many women who felt sick during their own pregnancy are feeling sympathy for the duchess, this condition is much more than just morning sickness. Hyperemesis gravidarum affects up to 2 percent of pregnant women.
The good news is that it doesn't last forever. Hyperemisis gravidarium usually ends after 20 weeks. However, hyperemesis gravidarum is an extreme condition that can be quite serious and potentially jeopardize both mother and baby if not monitored.
OB/GYN Dr. Yvonne Bohn said the cause is largely unknown. Some theories suggest a response to stress or hereditary reasons as factors. Most experts, however, believe it's due to fluctuating hormones during pregnancy.
"About 70 to 80 percent of women who are pregnant will experience some degree of nausea and vomiting, but hyperemesis is on the very extreme spectrum," Bohn said. "It's thought to be from the rising levels of estrogen and the hormone HCG in the body."
Nutrient deficiencies can also predispose a woman to hyperemesis gravidarum.
"Specifically, if someone has bulimia they might be more easily triggered into having vomiting in pregnancy, but there is no study that shows 100 percent that being thin or having eating problems predisposes them to hyperemesis," Bohn said.
Treatment often includes IV fluids, anti-nausea medication and vitamin therapy. Bohn often prescribes acupressure wristbands to help with the nausea.
Bohn said that although it's difficult to predict which women will get extreme morning sickness, study shows it's slightly more likely to happen to women who are expecting girls.