The major class of teratogens includes drugs and environmental chemicals. Some chemicals that are naturally found i the environment can cause birth defects. Skunk cabbage, when eaten, causes severe neurological damage. The picture at right shows the head of a cyclopic lamb born of a ewe which had eaten skunk cabbage early in pregnancy. Other substances derived from plants can also cause defects.
Alcohol can cause physical and mental retardation in the infant.
Alcohol seems to work by causing fetal cell death, impairing neural crest cell migration, and preventing cell adhesion. Nicotine and caffeine have not been proven to cause congenital anomalies, but women who are heavy smokers are likely to have infants that are smaller than those born to women who do not smoke. Smoking also significantly lowers the number and motility of sperm in the semen of males who smoke. Neural Tube Defects Dietary deficiencies can also lead to birth defects. Neural tube defects, such as failure to close the anterior or posterior neuropore, occur in about 1 in every 500 live births.