For each cigarette a pregnant woman lights up each day, she raises her baby’s risk of strabismus by 5 percent, new research suggests.

Further, the effect of smoking on the debilitating eye disorder may become more potent later in pregnancy.

“Although strabismus is a common condition, affecting some 2 to 3 percent of children, not much is known about its causes,” lead researcher Dr. Tobias Torp-Pedersen of the Statens Serum Institut in Copenhagen, Denmark, told Reuters Health by email.

Strabismus, often called “cross-eyes”, is a collective term for about 30 different conditions in which the eyes are unable to line-up in the same direction when focusing on an image. Some subtypes are linked with muscle problems, while others are linked to nerve damage. Left uncorrected, strabismus may lead to irreversible vision loss, in addition to psychological and social consequences.

Exposure in the womb to harmful substances may be one cause of strabismus. “Nicotine and other substances in tobacco, alcohol and caffeine all affect the brain in some way,” said Torp-Pedersen. “Minor disturbances to the developing brain could plausibly lead to strabismus.”

In fact, previous research had connected smoking and alcohol consumption during pregnancy with strabismus. But most of these studies were small and did not thoroughly investigate the timing and quantity of the exposures. The effect had also yet to be broken down by strabismus subtype, added Torp-Pedersen.

So he and his team identified more than 1,300 cases of strabismus from the Danish National Birth Cohort. All of the children were born between 1996 and 2003. Their mothers were interviewed twice during pregnancy, as well as at 6 and 18 months after delivery.

They found that women who smoked during pregnancy were at a 26 percent greater risk of having a child with strabismus compared to mothers who had not smoked, after adjusting for other risk factors such as alcohol consumption and the mother’s age.

The effect was not significant for smoking limited to the first trimester, but increased to 43 and 35 percent, respectively, for those who smoked during the first two or all three trimesters.

In their report in the American Journal of Epidemiology, the researchers also note that women smoking between five and nine cigarettes a day had a 38 percent greater risk of their baby developing strabismus compared to non-smoking mothers. Smoking 10 or more cigarettes a day was associated with a 90 percent greater risk.

“We were able to show that each extra cigarette smoked per day during pregnancy exerted a 5 percent increase in strabismus risk, which is a new finding,” Torp-Pedersen told Reuters Health.

He has this advice for pregnant smokers: quit smoking.

The effects were nearly the same between strabismus subtypes. And no added risk was seen with light smoking (less than five cigarettes a day), nicotine replacement therapy, or coffee or tea consumption.

Drinking small amounts of alcohol during pregnancy actually appeared to reduce the risk of strabismus. However, missing data may have distorted this result, the researchers caution. “We do not recommend consuming alcohol during pregnancy,” Torp-Pedersen emphasized.

Dr. Philip Lempert of Cornell University in Ithaca, New York, expressed concern that the study was unable to capture information about alcohol consumption during the critical first trimester, noting in an email to Reuters Health that “timing is the critical factor.”

SOURCE: American Journal of Epidemiology, March 25, 2010.