That offers important lessons for health professionals in the US and abroad, according to researchers from the Centers for Disease Control and Prevention (CDC). The researchers analyzed the smoking status of 187,302 women between the ages of 18 and 44 in 33 states from 1987 to 1996 using data obtained by telephone as part of the Behavioral Risk Factor Surveillance System (BRFSS).

The percentage of pregnant women who smoked dropped 4.5 percentage points between 1987 and 1996, from 16.3 percent to 11.8 percent. A drop in smoking by all women of childbearing age during that time ? from 26.7 percent to 23.6 percent ? revealed fewer pregnant women were smoking only because fewer women of childbearing age were taking up the habit, not because more were quitting when they found out they were pregnant.

The researchers found the data surprising. "We had expected that since the US has been at the forefront of anti-smoking initiatives over the last four decades and there has been a tremendous amount of information about why women shouldn?t smoke when pregnant or should quit when they realize they?re pregnant, that rates should really go down," said lead study author Shahul H. Ebrahim, MD, MSc, of the CDC.

"But when we looked at this nationally representative data from the largest survey in the world, it showed about 54 percent of smoking women continued to smoke after learning they were pregnant, a rate unchanged in the decade," said Dr. Ebrahim.

Health professionals should help women quit smoking, he said. "We have effective smoking intervention plans that should be made widely available to doctors, nurse-practitioners, midwives, and dentists. And they should be trained on how to use them. If we use this approach, we can reach many more women than we?re reaching now."

Pregnancy is an ideal time for such intervention, Dr. Ebrahim added. "Women have a natural biological distaste for harmful substances during pregnancy ? even coffee. And evolution has given women a tendency to avoid harming their babies."

Smoking by pregnant women is linked to an increased risk of miscarriage, stillbirth, premature delivery, and sudden infant death syndrome, and is a cause of low birth weight in infants. It also is a cause of lung and other cancers in mothers, as well as a host of other conditions including heart disease and emphysema, and is associated with reduced fertility and early menopause.

Dr. Ebrahim said the difficulty women have quitting when they realize they are pregnant, and relapse rates in general, point to how hard it is to quit smoking. Among the 34 percent of smokers who try to quit each year, less than 3 percent are able to do so permanently. That should serve as a warning to developing countries where tobacco use is increasing as those nations are targeted for sales growth by the tobacco industry, he added.

"Most important," said Dr. Ebrahim, "is for educators, public health officials, and the community to prevent young women from picking up smoking by all means. Only then can we achieve tobacco-free pregnancies in the future, because tobacco is very addictive and once you start, it?s very hard to give up."