The US FDA has assigned the pregnancy category B status to Benadryl, which means that it is considered safe to use, as there are no known side effects. However, your doctor will be the best person to judge whether or not you should take Benadryl while you are pregnant. He will also decide on the quantity of intake.
Human carcinogen -- Benzene: Benzene is a chemical deemed to be carcinogenic to humans. The carcinogenicity of the substance may be influenced by the nature (e.g. inhalation, ingestion, skin contact), duration and level of exposure. Benzene exposure is associated mainly with an increased risk of developing leukemia or lymphoma.
A benzene ring is a ring of six carbon atoms with three double bonds. The formula for benzene, the simplest arene and base structure for all others, is C6H6. Any compound with a benzene ring is called an aromatic compound. There are many such compounds, which tend to be carcinogenic.