Cigarette smoking during pregnancy may cause a variety of significant health problems for the baby, some of which may be fatal during infancy. Overall, babies born to smoking mothers are generally more likely to be significantly underweight and premature. In these cases, low birth weight may result from nicotine-induced maternal and fetal blood vessel constriction in both mother and baby, leading to decreased oxygen and nutrients provided to the fetus through the placenta. As with all premature infants, such babies born to smoking mothers may face a wide variety of complicated, serious health conditions due to their underdeveloped heart, lungs, and metabolism. However, smoker's premature babies may suffer not only from underdeveloped systems, but from actual permanent, structural airway and lung impairment as well. These impairments can easily lead to chronic childhood breathing problems, such as wheezing, asthma, and frequent, severe respiratory illnesses. Further, prenatal exposure to smoke may interfere with the development of fetal breathing control mechanisms and immune system regulation. Poor breathing control can aggravate any existing lung structure problems; poor immune system function may aggravate childhood respiratory problems, thus hampering the body's ability to recover from respiratory illnesses. Fortunately, all of these dangers may be avoided or at least partially alleviated when the mother chooses to quit smoking. Simply decreasing the number of cigarettes smoked per day doesn't seem to lead to significant improvement in fetal health. However, quitting within the first trimester of pregnancy can greatly reduce the chances of the baby being born with smoking-related health conditions.