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.
Benzene is a colorless chemical found naturally in the air and is one of the most frequently used chemicals in U.S. industry. Benzene enters the air through both natural occurrences, such as forest fires, and human activity, such as burning fossil fuels. Common sources of benzene that lead to human exposure include emissions from burning oil and coal, motor vehicle exhaust, and smoke from cigarettes.