What are Teratogens?A teratogen is an agent that can cause a birth defect. It is usually something in the environment that the mother may be exposed to during her pregnancy. It could be a prescribed medication, a street drug, alcohol use, or a disease present in the mother, which could increase the chance for the baby to be born with a birth defect. About 4 to 5 percent of birth defects are caused by exposure to a teratogen.When during pregnancy can teratogens really cause harm to the fetus?Teratogens are thought to have the ability to affect the fetus about 10 to14 days after conception. During the development of a baby, there are certain organs forming at certain times. There are some organ systems that are sensitive to teratogens throughout the entire pregnancy, such as the central nervous system. The central nervous system is the baby's brain and spine. One teratogen that affects the central nervous system is alcohol. Alcohol, at any time during the pregnancy, has the potential to cause birth defects and health problems in the baby, since the central nervous system is sensitive to teratogens the entire nine months of gestation. This is why alcohol consumption should be avoided in its entirety during pregnancy.There are three main teratogens that can really cause harm to the fetus. Fetal Alcohol Syndrome (FAS) Phenytoin (Dilantin) Varicella (Chickenpox)Fetal alcohol syndrome (FAS) is a group of abnormalities in babies born to mothers who consume alcohol during pregnancy. Fetal alcohol syndrome includes a characteristic group of defects including small head and brain, facial abnormalities, and defects of other organs.Phenytoin (or Dilantin) is a medication used to treat epilepsy (seizure disorder). With the use of this medicine there is a chance that the baby will be born with some of the following health problems: Growth deficiency Developmental delay Cleft palate Certain facial characteristics Heart defects Genitourinary abnormalities (abnormalities of the fingers and nails)Varicella (or chickenpox) is a highly infectious disease, usually associated with childhood.