Newborns of such mothers show lower levels of stress hormones and lowered stress response, the findings showed.

"Our results suggest that these newborns may not be mounting adequate hormonal response to daily stressors," said lead researcher Laura Stroud from The Miriam Hospital in Rhode Island.

"Their stress systems may not be prepared for the stressors of daily life," Stroud added.

The study included 100 mother-newborn pairs.

Results showed that infants exposed to smoking showed lowered levels of the stress hormone cortisol at baseline and in response to neuro-behavioural exams.

The lowered cortisol responses were consistent across seven behavioural exams over the first month of life.

Stroud also investigated the effects of smoking during pregnancy on DNA in the placenta, the temporary organ joining the mother and foetus during pregnancy.

"Our results suggest that effects of smoking during pregnancy on infant stress response are explained by changes in DNA," Stroud said.

"These alterations in stress hormones, stress response, and DNA may explain links between moms' smoking during pregnancy and the risk for their children to have behaviour problems and nicotine addiction in later life," Stroud concluded.

The study was published in the journal Psychoneuroendocrinology.



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