According to an article by Anahad O’Connor in the Health section of the New York Times (Jan 16, 2007), studies have shown that there is a lower rate of miscarriage (about 30% lower) among women who have nausea and vomiting in early pregnancy.  However, researchers have not found any relationship between morning sickness and other pregnancy outcomes, or figured out why it means miscarriage is less likely.

A widespread belief holds that morning sickness can be a sort of blessing in disguise, or at least a favorable sign. Many dismiss this notion as folklore, but studies suggest that the truth is not so clear-cut.

Studies have shown a lower rate of miscarriage among women with nausea and vomiting of any severity during pregnancy. The most recent, published in The International Journal of Obstetrics and Gynecology in 2006, found that of 7,000 women studied, those who had nausea in the first three months were far less likely to miscarry. That appeared to support a study by the National Institutes of Health that found that women who had morning sickness in the first four months of pregnancy were 30 percent less likely to miscarry.

The reasons are unclear. Increased nausea and vomiting are associated with higher levels of a hormone produced by healthy placental tissue, and one theory suggests that the sickness may help women avoid foods that could harm a developing fetus.

But many women have normal pregnancies with no morning sickness, and many miscarry without getting sick. And many studies have failed to find any relationship between morning sickness and other adverse outcomes, like stillbirth and birth defects.

THE BOTTOM LINE Morning sickness is associated with a lower rate of miscarriage, though it is not necessarily a sign of a healthy pregnancy.