What Are Teratogens and How Do They Affect Pregnancy?

Avoid These Agents for a Lower Miscarriage Risk and a Healthy Pregnancy

Pregnant woman smoking and drinking alcohol

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A teratogen is an agent that disrupts a baby's development when the mother is exposed during pregnancy. Known teratogens include toxic chemicals, radiation, viruses, alcohol, smoking, certain prescription drugs and numerous other agents.

What Is a Teratogen?

A teratogen is any agent that causes abnormalities when a developing baby is exposed to it during the mother's pregnancy. These environmental exposures can cause birth defects and other problems. Teratogens can have effects that range from mild to severe, and they are often most dangerous during early pregnancy and with excessive or prolonged exposure. There are several factors that influence the severity of the effect:

  • Type of Agent: What is the agent and what does it do to a developing fetus?
  • Dose: How much of the agent was the fetus exposed to?
  • Duration of Exposure: How long was the fetus exposed to the agent?
  • Time of Exposure: When in the pregnancy was the fetus exposed to the agent?
  • Genetics: Do the unique genetics of the mother or the fetus protect them or make them more susceptible to the agent?

Types of Teratogens

Perhaps the two most important teratogens today are alcohol and smoking. They are among the leading preventable causes of birth defects and developmental disabilities.

Other teratogenic agents include:

  • Drugs: Prescription, over-the-counter, or recreational 
  • Infectious Agents: Rubella, cytomegalovirus, varicella, herpes simplex, toxoplasma, syphilis, etc.
  • Physical Agents: Radiation, hyperthermia, etc.
  • Maternal Health Factors: Diabetes, maternal PKU, etc.
  • Environmental Chemicals: Organic mercury compounds, polychlorinated biphenyl or PCB, herbicides and industrial solvents, etc.

Risks of Consuming Alcohol During Pregnancy

Alcohol is one of the most concerning teratogens. It quickly passes from your blood to your baby's blood through the placenta and umbilical cord. Your fetus has the same blood alcohol level that you do if you drink, but it doesn't have the same ability to break down the alcohol.

Drinking during pregnancy can cause:

  • Miscarriage
  • Stillbirth
  • Preterm birth

Alcohol exposure during pregnancy is also the biggest non-genetic cause of mental retardation in the U.S. And if a fetus is exposed to alcohol - especially a lot or over a long period of the pregnancy - there is a risk of fetal alcohol spectrum disorders (FASD) or, worse, fetal alcohol syndrome (FAS). Fetal alcohol syndrome is marked by a pattern of birth defects including:

  • Small head and body size
  • Specific abnormal facial features
  • Learning and behavioral problems

Fortunately, you have complete control over your baby's exposure to alcohol during pregnancy. The Centers for Disease Control and Prevention advise that women who are having sex should stop drinking completely when they stop using birth control.

Risks of Smoking During Pregnancy

Smoking is another major teratogen. If you smoke during pregnancy, chemicals (nicotine, carbon monoxide, tar, etc.) pass into your developing baby's bloodstream, where they can deprive the fetus of oxygen it needs for proper development. There are many very serious harmful outcomes of smoking during pregnancy:

It's best to quit smoking before you become pregnant, but quitting at any time during a pregnancy can help your baby's health and the health of the pregnancy.

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  1. Mazzu-Nascimento T, Melo DG, Morbioli GG, et al. Teratogens: a public health issue - a Brazilian overview. Genet Mol Biol. 2017;40(2):387-397. doi:10.1590/1678-4685-GMB-2016-0179

  2. Gilbert-Barness E. Teratogenic Causes of Malformations. Ann Clin Lab Sci. 2010;40(2):99-114.

  3. Bailey BA, Sokol RJ. Prenatal alcohol exposure and miscarriage, stillbirth, preterm delivery, and sudden infant death syndrome. Alcohol Res Health. 2011;34(1):86-91.

  4. Vorgias D, Bernstein B. Fetal Alcohol Syndrome. Treasure Island, FL: StatPearls Publishing. Updated June 4, 2019.

  5. Centers for Disease Control and Prevention. More than 3 million US women at risk for alcohol-exposed pregnancy. Updated February 2, 2016.

  6. Kataoka MC, Carvalheira APP, Ferrari AP, Malta MB, de Barros Leite Carvalhaes MA, de Lima Parada CMG. Smoking during pregnancy and harm reduction in birth weight: a cross-sectional study. BMC Pregnancy Childbirth. 2018;18(1):67. doi:10.1186/s12884-018-1694-4

Additional Reading
  • Alcohol and Pregnancy Fact Sheet. MotherToBaby. May 2014.

  • Alcohol During Pregnancy. March of Dimes. July 2012.

  • More Than 3 Million US Women at Risk for Alcohol-Exposed Pregnancy. Centers for Disease Control and Prevention. February 2, 2016.

  • Smoking During Pregnancy. March of Dimes. December 2015.

  • Teratogens/Prenatal Substance Abuse. Understanding Genetics: A District of Columbia Guide for Patients and Health Professionals. Genetic Alliance; District of Columbia Department of Health. February 17, 2010.
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