TCM excels in preventing disease. The approach to preventing disease, including airbourne, is to strengthen the defensive chi. This can be done whilst pregnant, before or after. Pregnancy (in TCM terms) is actually a very healthy period of time in a womens life, and if she makes good choices regarding exposure of herself to the elements, physical and emotional stress and sick people, strong pollutants etc, the immune system will have no trouble dealing with seasonal pathogens. Your Dr. of Chinese Medicine can give you a diagnoses of whether or not your defensive qi is strong enough at any given time of year (your history will be taken into account), whether pregnant or not.