It's no secret that smoking while pregnant is pretty horrible for your health as well as your baby's. But I don't think anyone quite realized just how horrible it was for babies until a new study revealed that when it comes to the effects of smoking, the most damage occurs before a child is even born.

A new study from the University of California, San Francisco, aimed to determine when tobacco smoke exposure had the greatest effect on a child's health: before birth, from birth to age 2, or at the onset of asthma. Researchers found that exposure to tobacco smoke in utero was far more likely to cause severe asthma than exposure during the first two years of life. In fact, kids with severe asthma were more than three times more likely to have been born to mothers who smoked while pregnant than children with milder forms of the condition.

“There are environmental factors that leave their fingerprint on DNA and may have their expression several years out,” said Dr. Esteban Burchard, a professor of bioengineering and therapeutic sciences and medicine at UCSF and co-author of the study.

Despite warnings, nearly 14 percent of American women smoke while pregnant. 

Strong link between prenatal smoking and asthma
New study finds exposure to cigarette smoke has the greatest effect on a child's health when it occurs in utero.