Smoking declined significantly among American women of childbearing age between 1987 and 1996.
The Centers for Disease Control and Prevention found that the number of such women who had ever smoked declined from 44.1 percent to 38.2 percent.
The results of the telephone survey of 187,302 women ages 18 to 44 appear in Wednesday's Journal of the American Medical Association. The women surveyed were from 33 states.
The researchers, led by Dr. Shahul Ebrahim, said that women who said they were pregnant were about half as likely to be current smokers as those who were not -- a ratio that fluctuated only slightly over the 10-year survey period.
In 1987, 16.3 percent of the pregnant women reported being smokers and 26.7 percent of the non-pregnant women said they smoked.
By 1996, only 11.8 percent of the pregnant women smoked and only 23.6 of the non-pregnant women did.
Researchers said the decline in the number of pregnant women who smoke was probably related to fewer women taking up smoking in the first place, rather than more women quitting smoking because they were pregnant.
Their findings mirrored those of a CDC survey released in 1998, which showed similar percentage declines from 1990 to 1996 for pregnant and non-pregnant women in the same age range.
The researchers cautioned that because the latest survey excluded women without telephones, women younger than 18, and those who did not live in private residences, they may have underestimated the actual prevalence of smoking.