WASHINGTON — A paradoxical benefit of cigarette smoking during pregnancy finally may have been explained.
Smoking has long been linked to a decreased rate of preeclampsia. But rather than protecting against the disorder, smoking may mask the true incidence of preeclampsia by indirectly inducing preterm delivery, so that smokers' infants are simply born before preeclampsia can be manifested, Ahmad O. Hammoud, M.D., said at the annual meeting of the Central Association of Obstetricians and Gynecologists.
A 1999 study published in the New England Journal of Medicine found that the risk of developing preeclampsia was 32% lower in women who smoked than in nonsmokers. And most studies—a total of 99—cited in a review of the literature since 1959 also showed that smoking was associated with decreased risk. But this link “has always been questioned,” because it is counterintuitive that smoking could benefit pregnancy and because many of these studies had relatively small sample sizes, said Dr. Hammoud, a fourth-year resident in the department of ob.gyn. at Wayne State University, Detroit.
He and his associates examined the issue using a large German database of 170,254 singleton deliveries that took place at 29 hospitals across Germany during the late 1990s. Mean maternal age was 29 years. Overall, 23% of the women were smokers, and the overall rate of preeclampsia was 3.5%.
The incidence of preeclampsia was 2.5% among nonsmokers, compared with only 1.9% among smokers. Moreover, the incidence of preeclampsia showed a clear inverse correlation with the number of cigarettes smoked per day. Nonsmokers had the highest rate of preeclampsia, followed by women who smoked 1-5 cigarettes per day, then by women who smoked 6-10 cigarettes per day, and finally, by women who smoked more than 10 cigarettes per day.
“The new finding in our study was that the incidence of preeclampsia was not uniformly low in all smokers. It increased with advancing gestational age and was especially high in smokers who made it to 40 weeks or more,” Dr. Hammoud said.
“We postulate that placental damage from smoking leads to severe complications, such as placental abruption and restricted fetal growth, which in turn lead to preterm delivery before preeclampsia is manifested. So what smokers actually have is just an apparent decrease in preeclampsia,” he said.
This hypothesis is supported by the finding that smokers had a higher rate of placental abruption than nonsmokers and that fetal weight was adversely affected by smoking in a dose-response fashion, he noted.