Ask anyone who has ever been pregnant, and they will regale you with stories of nausea and vomiting during pregnancy – commonly known as “morning sickness.” Researchers estimate that between 50 to 90 percent of all pregnant women will experience some level of morning sickness in their first trimester. Some pregnant women are just slightly nauseous, while others have horrible bouts of vomiting and queasiness.

Morning sickness is one of the early signs of pregnancy, and it can hit women as early as 6 weeks pregnant. It typically starts to peak around 8 or 9 weeks pregnant, and for most women, it disappears completely by the second trimester. (That’s why the second trimester is called the “honeymoon” phase of pregnancy – all your horrible first trimester pregnancy symptoms go away!)

Unfortunately, for a small number of pregnant women, they continue to have severe morning sickness throughout all 40 weeks of pregnancy. This condition is called “hyperemesis gravidarum,” and it’s defined by extreme, persistent nausea and vomiting during pregnancy. In other words, you are constantly throwing up and feeling nauseous throughout your entire pregnancy. This condition can lead to dehydration, malnutrition, and significant weight loss. Approximately 60,000 pregnant women are hospitalized annual from this extreme form of morning sickness.

For years, the cause of hyperemesis gravidarum was largely unknown, though researchers had several theories. Many scientists believed that severe morning sickness was caused by the rapidly rising levels of the pregnancy hormone hCG (human chorionic gonadotropin), which is secreted by the placenta. That’s why women who carried twins are more likely to develop hyperemesis gravidarum.

According to a new study, published online today in the American Journal of Obstetrics and Gynecology, pregnant women whose sisters suffered from hyperemesis gravidarum are 17 times more likely to suffer from this debilitating morning sickness.

Researchers from the University of California – Los Angeles (UCLA) and the University of Southern California (USC) traced the family histories of women with hyperemesis gravidarum and discovered that this severe morning sickness had a strong genetic component.

Women who had sisters, mothers, and grandmothers on both sides of the family (paternal and maternal) were at a heightened risk of developing hyperemesis gravidarum during pregnancy. This was especially true if the pregnant women had sisters who suffered from the pregnancy complication.

The authors of this study looked at 650 participants. The pregnant women in the study had been diagnosed with hyperemesis gravidarum; the controls were women with at least two pregnancies that lasted longer than 27 weeks and who had not suffered from severe morning sickness. The researchers compared the family history of morning sickness in the pregnant women and the controls.

They discovered that the women with hyperemesis gravidarum with a sister who suffered from the same form of severe morning sickness were 17.3 times more likely to have the condition. In addition, 33 percent of the pregnant women reported having a mother who suffered from the condition, compared to only 8 percent of the controls.

When the researchers traced back to grandmothers’ pregnancies, they found that 18 percent of the pregnant women with severe morning sickness had a maternal grandmother with the same condition, and 23 percent of the pregnant women had a paternal grandmother – suggesting this condition is inherited through the father’s side of the family.

“Because the incidence of hyperemesis gravidarum is most commonly reported to be 0.5 percent in the population, and the sisters of cases have as much as an 18-fold increased familial risk for HG compared to controls, this study provides strong evidence for a genetic component to extreme nausea and vomiting in pregnancy,” the researchers concluded in the study.

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