The cause of severe morning sickness is unknown, although investigators have suspected there might be a genetic component. It occurs in up to 2 percent of pregnancies, and causes near-constant nausea and vomiting, putting both mom and baby at risk of serious complications.

To investigate what factors might affect the risk of severe morning sickness, Ase Vikanes, a PhD student at the Norwegian Institute of Public Health in Oslo, and colleagues looked at data on all births registered in Norway between 1967 and 2006. They identified nearly 550,000 pairs including a woman and her daughter and nearly 400,000 pairs including a woman and her son. All of the sons and daughters had children themselves.

One percent of the daughters of women who hadn't suffered from severe morning sickness had the condition, compared to 3 percent of the daughters whose mothers had suffered through it. The risk for the partners of men whose mothers had the condition was 1.2 percent.

This meant the women whose moms had severe morning sickness were three times more likely to develop the condition in their own pregnancies. Risk was still greater even if a woman's mother hadn't gotten sick while pregnant with her, but had been ill in previous or subsequent pregnancies.

These findings, the investigators say, suggest that there could be a genetic link from mothers to daughters for severe morning sickness. It's also possible, they note, that environmental factors shared by mother and daughter, such as nutrition or cigarette smoking, could be involved.

Decades ago, the researchers note, severe morning sickness was seen as a psychological problem, caused by a woman's "unconscious rejection" of the fetus or her partner. "Some women experiencing this condition are still told by their healthcare providers to 'quit pretending to be sick,'" they add.

However, Dr. Catherine Nelson-Piercy of Guy's and St. Thomas' Foundation Trust in London notes in an accompanying editorial, the psychological problems suffered by women with severe morning sickness are likely a consequence of the condition, not its cause.

The new findings should encourage research into the genetic causes of severe morning sickness, the researchers say. "This, as well as an understanding of the psychological consequences of experiencing severe nausea and vomiting, could be helpful for clinicians who treat and counsel women with" severe morning sickness, they conclude.