Benadryl is classified as a Category B medication by the U.S. Food and Drug Administration. This means that the medication has not caused harm to animal fetuses during studies, but that there have not been adequate studies to determine the impact on humans. Several studies have been done, but are not conclusive. One study showed that antihistamines in the last two weeks of pregnancy increase the risk of retinopathy for the developing fetus. Another study showed some potential for an increased risk of cleft palate for the developing fetus.
As for breastfeeding, Benadryl is not recommended for women who are breastfeeding as Benadryl passes to an infant via breast milk. Babies are sensitive to medications. Additionally, Benadryl may affect your milk supply, particularly if you are just starting to breastfeed. Benadryl can affect prolactin, which is important for the production of breast milk.