NEW YORK (Reuters Health) - Preemies whose mothers smoked during pregnancy have a tougher time recovering from low-oxygen conditions, new research shows. The findings may help explain why infants born early to smokers are at greatly increased risk of sudden infant death syndrome (SIDS).

Pregnant women who smoke are about 30 percent more likely to give birth prematurely, Dr. Shabih Hasan from the University of Calgary in Alberta noted in an interview with Reuters Health, while prematurity itself increases SIDS risk.

Furthermore, now that babies are routinely put to sleep on their backs, cigarette smoke exposure in the womb and in early infancy has become the chief risk factor for SIDS, Hasan and his colleagues point out in the American Journal of Respiratory and Critical Care Medicine.

Preemies' breathing control systems have not developed fully, and these infants frequently have apneas, or breathing pauses, Hasan and his team write. To examine the effect of prenatal smoke exposure, the researchers looked at the breathing responses of 22 infants when oxygen levels dipped. Twelve of the infants were born to women who smoked at least five cigarettes daily.

The researchers observed the babies for three hours, and then reduced the oxygen concentration of the air the infants were breathing for five minutes. They were monitored closely, and the test was halted if the oxygen level in their blood fell too low.

Before the test, the smoke-exposed and non-exposed infants showed identical breathing patterns. However, the heart rates of the babies born to smokers jumped more sharply during the low-oxygen condition, and didn't return to pre-challenge levels, suggesting they were more stressed by the lack of oxygen.

Also, blood oxygen levels in these babies didn't return to normal as quickly as it did in the non-smoke-exposed babies.

"These preterm babies have difficulty in recovering from a low oxygen episode," Hasan told Reuters Health. "One can speculate that this could be the underlying mechanism for these babies to have SIDS."

If this is found to be the case, he added, a low-oxygen "challenge" could be used to identify infants at risk of SIDS before they leave the hospital, so they could be monitored more closely.

The findings underscore the importance of smoking prevention, especially in adolescence, which is when most people pick up the habit, Hasan said.

He also called for offering more help to women who smoke and have difficulty quitting in pregnancy. "I don't think there's any mother who's going to say, 'No I'm going to keep smoking and hurt my baby.' They don't know where to turn," said Hasan, adding that doctors and health care professionals often fail to ask pregnant women if they smoke, and if so how much.

SOURCE: American Journal of Respiratory and Critical Care Medicine, September 2008.