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 carcinogen is a substance that is capable of causing cancer in humans or animals. If a substance is known to promote or aggravate cancer, but not necessarily cause cancer, it may also be called a carcinogen. Though there are many things that are believed to cause cancer, a substance is only considered carcinogenic if there is significant evidence of its carcinogenicity. A carcinogen may act on deoxyribonucleic acid (DNA), causing dangerous changes, or it may work to increase the rate of cell division.