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Being stressed during pregnancy may increase the risk of children becoming smokers.

Previous studies found that smoking during pregnancy increases the risk of children taking up the habit. Female babies exposed to nicotine and maternal stress hormones in the womb are more likely to smoke later in life, according to research.

However, researchers did not find the same effect in male babies.

Researchers said the latest findings suggest that sex hormones may explain the higher risk of smoking in female fetuses exposed to maternal stress.

"Our findings highlight the particular vulnerability of daughters to long-term adverse outcomes following maternal stress and smoking during pregnancy," said Dr. Laura Stroud, of The Miriam Hospital, Rhode Island, according to a news release.

"We do not yet know why this is, but possible mechanisms include sex differences in stress hormone regulation in the placenta and adaptation to prenatal environmental exposures," Stroud added. "Also, cortisol and nicotine may affect developing male and female brains differently.  If daughters of smoking mothers are more likely to grow up nicotine dependent, the result is a dangerous cycle of intergenerational transmission of nicotine addiction."

The latest study involved 1,086 pregnant women enrolled in a large, national, long-term health project that began in 1959. Researchers measured the women's cortisol and testosterone levels and recorded their smoking status. The study involved 649 daughters and 437 sons who were followed for the next four decades.

"While maternal smoking during pregnancy has been shown to be an independent risk factor for nicotine dependence, we did not really know which pathways or mechanisms were responsible. Most prior research involving biological mechanisms had been conducted in animals not humans," said Stroud.

"Our study suggests that maternal smoking and high stress hormones represent a 'double-hit' in terms of increasing an offspring's risk for nicotine addiction as an adult. Because mothers who smoke are often more stressed and living in adverse conditions, these findings represent a major public health concern," she added.

The findings revealed exposure to nicotine and stress hormone cortisol increased the risk of smoking in female but not male offspring.

"This new data may help us focus our attention on individuals at greatest risk for later smoking," Dr John Krystal, editor of Biological Psychiatry, said in a news release. "It is interesting female, but not male, offspring seemed to be at greatest risk. Sex differences in the vulnerability to smoking are important and merit further study."

The findings are published in the journal Biological Psychiatry.