Overall, about one in four women who smoke while pregnant deny it, a new study hints. The numbers could be even higher in certain groups of women, like those in their early 20s.

In the United States, smoking by moms-to-be is one of the most common preventable causes of illness and death among infants, Dr. Patricia Dietz from the division of reproductive health at the Centers for Disease Control and Prevention in Atlanta, and colleagues note in their report.

In their study, they estimated how many pregnant and nonpregnant smokers aged 20 to 44 years did not disclose their habit on a health questionnaire.

How did they catch the deception? They took blood samples from the women to measure levels of cotinine - a byproduct of nicotine that serves as a marker of exposure to tobacco smoke. Their analysis included 994 pregnant women and 3,203 nonpregnant women.

Overall, 13 percent of pregnant women and 30 percent of nonpregnant women were active cigarette smokers. The pregnant smokers smoked an average of 11 cigarettes a day, while the nonpregnant smokers averaged close to 14 cigarettes a day.

According to the investigators, far more pregnant than nonpregnant smokers failed to disclose their habit - 23 percent versus nine percent - and were identified by their cotinine concentrations.

For a variety of reasons, such as the fact that pregnant women’s bodies break down cotinine faster, the researchers think the results “likely underestimate” the true number of pregnant women who smoke and don’t say so.

Among both pregnant and nonpregnant smokers, those most likely to keep this information to themselves were women aged 20 to 24, as well as those with Medicaid or other source of government-funded health insurance and those with less than a high school education.

Race also factored in. Pregnant smokers who failed to report their habit were most likely to be non-Hispanic black. In women who weren’t pregnant, nondisclosure was most common among Mexican-American women and non-Hispanic blacks.

Writing in the American Journal of Epidemiology, Dietz and her colleagues say their findings may have important implications for researchers studying how smoking during pregnancy affects the developing baby, as roughly one in four pregnant smokers, and one in 10 nonpregnant smokers, deny smoking.

Studies and surveillance systems that rely on people to accurately state their smoking status may get inaccurate information, “especially among pregnant women,” they warn.

SOURCE: American Journal of Epidemiology, Published online December 22, 2010.