April 21, 2010

Smoking May Reduce Pregnancy Complication

A new study has found evidence that women who smoke during pregnancy have a lower risk of a complication called preeclampsia.

Researchers say that the results do not detract from the importance of women quitting the habit before they try to become pregnant. Smoking during pregnancy is linked to increased risks of miscarriage, low birth weight and other pregnancy complications.

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

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

A number of studies have linked smoking to a reduced risk of the condition, but so far there has not been a connection made to understand why.

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

However, there was no protective effect seen among pregnant women who used "snus," which is a type of smokeless tobacco popular in Sweden.

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

She said that a byproduct of burning tobacco, possibly carbon monoxide, might be the cause.

The team reported the findings in an American Heart Association journal called Hypertension.

Even though the reasons for tobacco smoke's effectiveness on preeclampsia remains unclear, carbon monoxide has been shown to lower levels of certain proteins that inhibit new blood-vessel formation, including one known as sFlt1.  This protein is known to rise in the weeks before preeclampsia develops. So, according to Wikstrom, it is possible that smoking lowers preeclampsia risk by keeping sFlt1 levels down.

The team based their findings on information gathered from Sweden's national birth register. Among the women who were heavy smokers during pregnancy, 1.7 percent developed preeclampsia. Three percent of the non-smoking women developed the condition while 2.3 percent of the light smokers did.

Once the researchers accounted for several variables in preeclampsia risk, like 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 had a 49 percent lower risk than non-smokers.

Wikstrom said 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 told Reuters.

"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."