TERATOGENS

A teratogen is any chemical, substance, or exposure that may cause birth defects to the developing fetus. Please note that no links are provided below. We recommend that you visit the March of Dimes Resource Center for further information about any of the exposures listed below.

 General Features 

• Dosage and Threshold Effects:  A teratogen can cause an effect only after a certain level of exposure is reached.
• Timing:  The effect of teratogens depends upon the timing of exposure.  The first trimester of pregnancy is the critical period of organ and limb development in the fetus.  The fetal brain develops throughout pregnancy and can be affected at any time. Exposure to a teratogen during the two weeks following conception is unlikely to cause birth defects.
• Although teratogens may increase the risk for birth defects, they do not necessarily cause problems in all cases.
• 2-3% of all babies are born with some type of birth defect.  Exposure to a teratogen may increase this risk, but nothing can eliminate the risk.

Some Common Teratogens (Not a Comprehensive List)

Congenital Infections

• Cytomegalovirus (CMV): CMV is transmitted by sexual contact and secretions.  Adult infections may be asymptomatic.  About 1% of pregnant women are infected, and only a small percentage of these will have babies with clinical symptoms, which include hearing loss, microcephaly, mental retardation, visual defects and dental anomalies.
• HIV: The biggest risk to the fetus is for transmission of the virus to fetus, not for birth defects or mental retardation.  Treatment during pregnancy can greatly reduce the risk to the baby.
• Rubella:  Rubella is transmitted by contact with nasal secretions of infected individuals.  Infected pregnancies are at increased risk for miscarriage.  Infection during the first trimester results in a high rate of birth defects in the baby.  This risk decreases sharply with infection in the second and third trimesters.  The birth defects associated with congenital rubella syndrome include eye defects, hearing loss, heart defects, mental retardation, and growth retardation.
• Syphilis:  This is a sexually transmitted disease.  Half of infected pregnancies will spontaneously abort.  Not all infected pregnancies will show symptoms.  Symptoms of congenital syphilis infection include prematurity, enlarged liver and spleen, anemia, jaundice, skin rash, mental retardation, blindness, and deafness.
• Toxoplasmosis:  This infection is transmitted by contact with raw or undercooked pork, or with cat feces.  About 75% of infected fetuses will not have symptoms.  Symptoms can include prematurity, low birth weight, enlarged liver and spleen, visual defects, mental retardation and intercranial calcifications. Treatment during pregnancy can improve outcome.
• Varicella:  Varicella is transmitted by airborne respiratory droplets and direct contact with an infected individual's rash.  Babies infected before 20 weeks of pregnancy have an approximate 2% risk of being affected with congenital varicella syndrome.  This syndrome can include skin scarring, defects of the muscle and limbs, a smaller head, blindness, seizures, and mental retardation.  Exposure at around the time of delivery causes infection in about 25% of exposed newborns.  Infected newborns develop a rash and can die if not treated.

Chemicals, Drugs and Medications:

• Alcohol: The features of fetal alcohol syndrome (FAS) occur in 30-40% of babies born to chronic alcoholics.  Another 50-70% of such babies may suffer from fetal alcohol effects (FAE), which is milder than FAS.  To be diagnosed with FAS a baby must have the following features:
  • growth retardation
  • central nervous system problems
  • characteristic facial appearance (small head, flat facial profile, thin upper lip)
  • other major birth defects (heart, gastrointestinal, etc) are possible
  • Children with FAS have failure to thrive, mild to moderate mental retardation and behavior problems.  FAS is the most preventable cause of mental retardation.
• Thalidomide:  Well known teratogen leading to limb defects in exposed babies.  This medication to prevent morning sickness is not used anymore.
• Cigarette Smoking:  Constant exposure to cigarette smoke decreases the amount of oxygen crossing the placenta and can lead to low birth weight and premature babies.
• Cocaine, Street Drugs:  Women addicted to street drugs are at increased risk for having babies born addicted to these drugs.  The biggest risks to the babies are not from the drugs themselves, but due to poor maternal nutrition and lack of prenatal care. Prenatal cocaine exposure is not associated with a syndrome (a group of seemingly unrelated birth defects found to have a single cause).  Studies are still on-going, looking at long term effects of exposures in school age children.  Cocaine is a vasoconstrictor - causes blood vessels to shrink.  There are many prenatal complications due to even one exposure to cocaine:
  • miscarriage
  • placental abruption
  • vascular defects (limb reduction)
  • in utero strokes
  • increased risk for SIDS
  • infant drug withdrawal
• Anticonvulsants:  Exposure to antiseizure medications is associated with fetal anticonvulsant syndrome.  Approximately 10% of children exposed to Dilantin and other seizure medications will have some growth and mental retardation, digit hypoplasia (shortening), and a characteristic face.  Use of valproic acid and carbamazepine in early pregnancy are associated with a 1% risk for spina bifida.  
• Lithium:  Exposure to lithium is associated with an increased risk for congenital heart defects Retinoids, Vitamin A (Accutane):  Women taking oral Accutane have a high risk of having a baby with major birth defects including hydrocephaly (excess fluid in the brain), small head size, mental retardation, malformed ears, facial abnormalities, and heart defects.  Topical Retin-A has not proven  to increase the risk for birth defects.

Physical Agents

• Diagnostic X-rays:  Routine x-ray exposure (dental x-rays, diagnostic x-rays) is not generally thought to cause birth defects.  Large exposures, such as those seen in radiation therapy for cancer may cause miscarriage early in pregnancy.

Maternal Factors

• Maternal Diabetes:  Elevated glucose levels in early pregnancy are associated with an increased risk for birth defects.  If diabetic control is poor, the risk is up to 25% for birth defects including cardiac defects as well as spine and limb abnormalities.  Preconceptual control of glucose levels can significantly reduce the risk for birth defects.
• Maternal PKU:  Women with PKU who do not maintain phenylalanine restricted diets have a 90% risk to have a child with small head size and mental retardation.  Diet restriction prior to and during pregnancy can greatly improve outcome. 

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