Pregnant women who suffer morning sickness are less likely to miscarry, say U.S. researchers.

In fact, in a study published in a recent issue of the journal of Human Reproduction, the longer participants experienced nausea and vomiting, the lower their miscarriage risk, especially if the woman was older.

Women who don't have morning sickness shouldn't be alarmed, however, says one of the study's authors, Dr. Ronna Chan of the University of North Carolina at Chapel Hill.

"Not all pregnant women who go on to have successful pregnancies experience nausea and vomiting early on or at all," she told Reuters Health. "In addition, pregnancy symptoms can vary from one pregnancy to the next, even for the same woman."

Earlier studies have also suggested a link between morning sickness and reduced risk of miscarriage, but the researcher say their study is the first to look at duration of symptoms.

They followed 2,407 pregnant women in three American cities between 2000 and 2004 through interviews, ultrasound assessments and medical records.

They found women who didn't experience morning sickness during their first trimester were 3.2 times more likely to miscarry, compared with those who did have morning sickness.

Furthermore, women over the age of 35 who experienced nausea and vomiting for half their pregnancy were 80 per cent less likely to miscarry compared to those in their age group who didn't have morning sickness.

It's not clear why there is a link, but some researchers suggest vomiting and nausea are a sign of sensitivity to the increase in hormones that help a woman stay pregnant.