The human brain is one of the most complex organs that have mystified scientists. Even with all of the strides that science has made, there is still a great deal about the human brain that leaves scientists perplexed. There has never been a doubt as to the complexity of the human brain and its vast abilities. Of all the stages of brain development, infancy has to be one of the most critical of the various stages. When a child is conceived, by the eighth week of pregnancy (of a forty week duration) the fetus’s brain has already begun to form and send off signals and directions to the rest of the undeveloped body. During this prenatal phase of development the brain forms billions of neurons. These neurons are nerve cells that deal with the information that is processed and transmitted from the brain. In this stage the neurons are positioning themselves in their appropriate places. This is a very delicate stage of development for a developing and maturing brain. There are a variety of outside agents that can act as a hindrance in this process. Teratogens are agents that can cause birth defects or even worse, loss of the fetus. To measure the severity of a particular teratogen, scientists must look at the dose, genetic susceptibility and time of exposure. The dose pertains to the amount of the agent that has been consumed. For example, if significant amounts of coffee are consumed on a daily basis by an expectant mother, then it is more likely the caffeine would have an ill effect on her unborn child. Another tool used in measuring the severity of teratogen is genetic susceptibility. Genetic susceptibility identifies how severe the abnormalities are that have been caused by the teratogens. For instance, there can be two expectant mothers that both consume considerable amounts of coffee during the duration of their pregnancy. But because of their individual make R.Bernard2 up, one of the expectant mothers may metabolize the caffeine differently. While one fetus may suffer from the ill effects of the teratogen, the other fetus may be completely unharmed. Time of exposure is also a means of measuring the influence of a teratogen. The length of time deals with the stage of development that the teratogen was introduced to the unborn child and for how long. Certain exposures of teratogens have more of an adverse effect on a developing baby than others. There are several types of teratogens that can affect the brain development of a new born child such as alcohol, nicotine, stress and maternal diseases such as genital herpes, syphilis and HIV. Alcohol can cause Fetal Alcohol Spectrum Disorders (FASD). Newborns with FASD can be born with difficulty learning and even end up mentally retarded. With FASD, the child has the ability to end up with distorted facial features or many other developmental issues while growing up. Nicotine can also contribute to affecting fetal brain development. Women that insist on smoking during their pregnancy not only run the risk of having a child with low birth weight, but the child may also be born with Attention Deficit Hyperactivity Disorder (ADHD). This is a disorder that is very common among this current generation of children. Stress is another agent that have a great deal of effect on the brain development of an unborn child. Pregnant women who suffer from great deals of stress or anxiety are more susceptible to having children born with emotional or cognitive issues, language delay or ADHD. Although stress for anyone is a hazard, it is advised that expectant mothers tyr to avoid stress or stressful activities not only for their health, but for the health of their child. R.Bernard3 The effects of teratogens are also equated with maternal diseases. Genital herpes is a sexually transmitted disease that is treatable, but not curable. Women that have genital herpes run the risk of their baby being born with brain damage. This can be catastrophic