Ase Vikanes of the Norwegian Institute of Public Health found women born in India and Sri Lanka were three times more likely than ethnic Norwegians to suffer from extreme nausea and vomiting in pregnancy -- known as hyperemesis gravidarum.

A higher prevalence of hyperemesis gravidarum was also associated with maternal age between 20 to 24 years old, being married and carrying a female or more than one fetus.

Vikanes and colleagues used data from the Medical Birth Registry of Norway, showing 8,300 cases of hyperemesis gravidarum recorded out of 900,000 pregnancies over 40 years -- an overall prevalence of 0.89 percent. The mother's country of birth and education -- recorded by Statistics Norway -- were linked through the mother's personal identification number.

"In contrast to earlier studies we tested the quality of the data and therefore have confidence in our findings" Vikanes said in a statement. "The difference in prevalence of hyperemesis gravidarum related to the mother's country of birth cannot be explained by differences in socio-demographic characteristics."

More than 90 percent of pregnant women experience some degree of nausea and vomiting, but only 0.5 percent to 2 percent have hyperemesis gravidarum.