There is no safe level of tobacco use among pregnant women and a direct correlation exists between the number of cigarettes smoked and poor outcomes. Even if it is only one a day, smoking during pregnancy puts mom and baby at risk. It increases the risks of birth defects and causes issues during pregnancy including miscarriage and stillbirth. Smoking can also decrease the chances of conceiving.
For more information on the risks of smoking, visit https://www.babycenter.com/pregnancy/health-and-safety/how-smoking-during-pregnancy-affects-you-and-your-baby_1405720.