Researchers found that women were more likely to experience a serious form of morning sickness if their mothers or sisters did as well.
Looking specifically at a very severe form of nausea known as hyperemesis gravidarum (HG), the authors found that women with sisters who had HG were 17 times more likely to also develop HG. Women with this condition have unrelenting, excessive nausea and vomiting that puts them at risk of malnutrition, dehydration and significant weight loss.
Study author Dr. Marlena Fejzo of the University of Southern California-Los Angeles told Reuters Health she wasn't surprised by the findings, since previous research has shown that severe morning sickness is more likely to affect both members of identical twin pairs, hinting at a heritable element.
However, in the new study, she added, “the degree of heredity is very exciting because it suggests genes are involved, and when we find those genes, we may finally understand the cause of severe nausea in pregnancy and be able to make new treatments that are designed to treat the cause rather than the symptoms.”
Most pregnant women - an estimated 75 percent - experience some morning sickness, according to the American Pregnancy Association, but 1 percent suffer the extreme HG form of illness that can require hospitalization.
It's unclear why some women become nauseous while pregnant and others don't. Even animals such as dogs and monkeys appear to experience a form of morning sickness, Fejzo noted. “There are even reports of snakes avoiding food during pregnancy,” she said in an e-mail.
To investigate whether severe forms of nausea might have genetic roots, Fejzo and her team reviewed information collected from 207 women who experienced HG during pregnancy and had at least one sister who had also been pregnant. They compared their responses to 110 of the patients' female friends who had relatively nausea-free pregnancies, serving as controls.
The researchers found that 14 percent of women who experienced HG during pregnancy had sisters who also had HG, versus less than 1 percent of women who did not have HG.
When combining HG with other severe forms of morning sickness - persistent nausea that was not bad enough to require IV fluids or nutrition - a family history also appeared to put women at higher risk. Specifically, 34 percent of women with HG also had an affected sister, versus 8 percent of women who were never diagnosed with HG.
Among 469 women with HG and 216 of their female friends, 33 percent of those with HG had a mother with severe nausea or HG as well, versus only 8 percent of their friends.
“There can be variation in nausea and vomiting from one pregnancy to the next, which suggests that not only genes are involved but also other factors,” Fejzo noted. “For example, some studies suggest a female fetus or carrying multiple fetuses results in more nausea. So I would speculate that the level of nausea in pregnancy is a combination of both genetic factors and non-genetic factors.”
One concern about these findings, noted Dr. Andrej Grjibovski at the Norwegian Institute of Public Health, who did not participate in the study, is that women with HG might have been more likely to volunteer to participate in the study if they had relatives who were also affected. And since these women recruited the controls themselves, they “may not be representative of the general population,” he cautioned.
Still, Grjibovski said in an e-mail that he was “not at all” surprised by the findings, since other research has suggested both maternal and paternal genes may play a role in HG. A recent analysis of more than 2 million birth records showed that women whose mothers suffered from a serious type of morning sickness were at triple the risk of the condition themselves.
HG hospitalizes more than 59,000 women every year in the U.S. A recent review of 27 large studies concluded that there is no reliable treatment for nausea in early pregnancy. Still, options include dietary changes (such as eating small meals and avoiding spicy foods), alternative therapies such as acupressure and hypnosis, and some prescription anti-nausea medications.
Fejzo said her team is currently planning a study to compare the genes of 1,000 women with HG to those of 1,000 of their unaffected friends. “With this approach, we should be able to identify the genetic variants that predispose to HG and then hopefully create new, more effective medications that are designed to correct the cause of the disease as opposed to the current medicines which are used to treat the symptoms.”
Source: American Journal of Obstetrics & Gynecology, online October 25, 2010.