Secondhand smoke does not necessarily cause fetal malformation, but usually only increases the chance of fetal malformation. Because of the nicotine in cigarettes, if pregnant women smoke secondhand smoke for a long time, it can cause vasoconstriction of the placenta, resulting in reduced blood supply to the fetus in the uterus, leading to fetal distress; it can also have adverse consequences on fetal development, such as mental retardation and developmental malformations. However, these conditions do not necessarily lead to fetal malformations, and if the screening tests show normal results, there is nothing to worry about. However, it is still recommended that pregnant women should minimize their exposure to second-hand smoke and open windows frequently to keep the air fresh. In addition, pregnant women should try to eat more vegetables and fruits containing folic acid and take small doses of folic acid during early pregnancy to prevent the occurrence of fetal neural tube abnormalities.