Some doctors say stress reduction - or help coping with extreme circumstances - should be added to the list.
They found premature birth was more than twice as likely if severely stressful events occurred during the first and second trimesters, but not the third. Low birth weight was nearly three times more likely if stress happened during the first trimester.
Stress during pregnancy causes higher levels of cortisol and adrenaline in the fetus' bloodstream. After 9/11, pregnant widows were offered community help to cope with their grief and prepare for delivering a baby who would never know his or her father.