A teratogen is a chemical or infectious agent that can cause birth defects by adversely altering the development of an embryo or fetus without necessarily altering the organism's genetic structure. Some substances will only act as teratogens at specific phases of an embryo's or fetus' development and may cause little or no harm if it is exposed to the same substance at a different phase.
For instance, the sedative drug thalidomide can interrupt the development of the long bones of the arms and legs in human embryos, resulting in defects such as children born with no arms but hands growing directly from their shoulders. This drug also causes heart defects, intestinal deformities, and a lack of external ears. However, the drug was found to only affect embryos 20 to 36 days after conception; older fetuses were not affected.
If a first-trimester fetus is exposed to rubella (German measles), the disease can cause the child to have eye defects, partial or complete deafness, heart defects, mental retardation and sometimes cerebral palsy.
Alcohol can cause mild facial defects, mental retardation, and other symptoms characteristic of fetal alcohol syndrome.
Other known human teratogens include:
Contrast teratogens with mutagens, which can cause birth defects by creating mutations in an embryo's genetic structure.
Reference: http://www.devbio.com/chap21/link2110.shtml. Also based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/ | 
