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.
For practical reasons, the drinking water guideline for benzene is 1 ppb (which is the analytical detection limit) (NHMRC 2004). The remaining chemicals (toluene, ethylbenzene and xylenes) are not recognized as carcinogenic. (IARC 1989) and their drinking water guidelines are much higher (Table 4).