A teratogen is an agent that can cause or increase the chance of 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, chemical exposure, or an infection present in the mother, all of which could increase the chance for a baby to be born with a birth defect.
Once the egg is fertilized (conception), it takes about six to nine days for the egg to implant in the uterus. Once the fertilized egg is attached to the uterus, a common blood supply exists between the mother and the embryo. If something is in the mother's blood, it can now cross over to the developing fetus. Teratogens are thought to have the ability to begin affecting the fetus about 10 to14 days after conception.
During the development of a baby, there are certain internal organs forming at certain times. For example, if a teratogen has the potential to interfere with the closure of the neural tube the exposure to the teratogen must occur in the first 3.5 to 4.5 weeks of the pregnancy (since this is when the neural tube closes in a fetus). There are some organ systems that are sensitive to teratogens throughout the entire pregnancy, such as the central nervous system. The central nervous system includes the baby's brain and spine. One teratogen that affects the central nervous system is alcohol. Alcohol use, at any time during the pregnancy, has the potential to cause birth defects and health problems in a baby, since the central nervous system is sensitive to teratogens the entire nine months of gestation. This is why alcohol consumption should be completely avoided during pregnancy.