A new study adds to evidence that women who smoke during pregnancy have a lower risk of a complication called preeclampsia - in findings that may help shed light on how the condition arises.

The results, researchers say, do not in any way detract from the importance of women quitting the cigarette habit, ideally before they try to become pregnant. Smoking during pregnancy is linked to increased risks of miscarriage, low birth weight and other pregnancy complications.

But if researchers can gain a better understanding of why smoking is related to a lower risk of preeclampsia, it could lead to new ways to treat or prevent the complication, according to Dr. Anna-Karin Wikstrom, the lead researcher on the new study.

Preeclampsia is a syndrome marked by a sudden increase in blood pressure after the 20th week of pregnancy and a buildup of protein in the urine. Left untreated, it can develop into a life-threatening condition called eclampsia, which can cause seizures or coma.

A number of studies have linked smoking to a reduced risk of preeclampsia, but the reasons for the connection have not been clear.

In the new study, Wikstrom and her colleagues found that among more than 600,000 Swedish women who gave birth between 1999 and 2006, those who smoked during pregnancy were one-third to one-half less likely to develop preeclampsia as non-smokers.

In contrast, there was no protective effect seen among pregnant women who used “snus” - a type of smokeless tobacco popular in Sweden.

Because both cigarettes and smokeless tobacco contain nicotine, the findings suggest that nicotine is not the reason for the lower preeclampsia risk, Wikstrom told Reuters Health in an email.

Instead, she said, a byproduct of burning tobacco - possibly carbon monoxide - may be at work.

Wikstrom and her colleagues at the Karolinska Institute in Stockholm, Sweden, report the findings in Hypertension, an American Heart Association journal.

The exact reasons why tobacco smoke would offer protection against preeclampsia remain unclear. However, carbon monoxide has been shown to lower levels of certain proteins that inhibit new blood-vessel formation, including one known as sFlt1. Levels of this protein, in turn, are known to rise in the weeks before preeclampsia develops. So it’s possible, according to Wikstrom, that smoking lowers preeclampsia risk by keeping sFlt1 levels down.

She and her colleagues based their findings on information from Sweden’s national birth register, focusing on all 612,712 singleton births in the country between 1999 and 2006. Among women who were heavy smokers during pregnancy - smoking 10 or more cigarettes per day - 1.7 percent developed preeclampsia; the corresponding rates were 2.3 percent among light smokers and 3 percent among non-smokers.

After the researchers accounted for several factors in preeclampsia risk - such as the mother’s age and weight in early pregnancy - light smokers were one-third less likely to have developed preeclampsia than non-smokers. Heavy smokers, meanwhile, had a 49 percent lower risk than non-smokers.

Wikstrom stressed, however, that even if smoking is responsible for the lower risk, the findings should not deter women from quitting.

“Cigarette smoking during pregnancy is known to increase the risk of a number of adverse outcomes, such as miscarriage, fetal growth restriction and preterm birth,” she said.

“Although our findings suggest that smoking might reduce the risk of preeclampsia, the adverse effects of smoking on the fetus outweigh by far this possible benefit.”

SOURCE: Hypertension, May 2010.