Antihistamines may be used for the treatment of allergic rhinitis, upper respiratory infections, urticaria/angioedema, atopic dermatitis, and, rarely, as adjunctive treatment for anaphylaxis, during pregnancy. Because these illnesses may affect maternal comfort and safety as well as threaten the fetus directly (anaphylaxis) or indirectly, they often require therapy during pregnancy. Based on the information available to date, in this chapter we have attempted to provide rational guidelines for the gestational use of H1-receptor antagonists in a manner that will lead to the optimal well-being of both the mother and her infant. As more information becomes available, the recommendations herein may require modification. Although this chapter has dealt specifically with gestational management, a case can be made for considering this information when making therapeutic decisions in all women of childbearing potential. First, most pregnancies are unplanned, and the peak period of fetal vulnerability to drug-induced teratogenesis begins the day a woman's period is due. Second, during gestation, substantial alterations in a previously successful but not optimal-for-pregnancy chronic therapeutic regimen may be psychologically threatening to the patient and may lead to either uncontrolled disease or unanticipated side effects. Thus, pregnancy-appropriate regimens should ideally be discussed with all women of childbearing age as part of the informed therapeutic decision-making process.