This chapter discusses the most common genetic factors known to affect prenatal growth and development. The factors influencing prenatal life are believed to result from genetic or environmental influences, and they have either a positive or negative effect. Prenatal development of the human organism can be described in three major phases: the germinal period, the embryonic period, and the fetal period. Certain drugs and medications can enter fetal circulation; therefore, pregnant women should take them only as advised by a physician. Recreational drugs, prescription drugs, nonprescription drugs, and obstetrical medications are all chemicals that can affect both the mother and the developing fetus. Narcotics are a class of drugs frequently used to treat acute and chronic pain. However, this class of drug can also produce a feeling of relaxation, as well as producing a “high.”.