A teratogen is a substance that interferes with normal fetal development and causes congenital disabilities. Drugs, alcohol, chemicals and toxic substances are examples of teratogens. Teratogens can also increase the risk for miscarriage, preterm labor or stillbirth.

What are teratogens?

Teratogens are substances that cause congenital disorders in a developing embryo or fetus. A teratogen is anything a person is exposed to or ingests during pregnancy that’s known to cause fetal abnormalities. Drugs, medicine, chemicals, certain infections and toxic substances are examples of teratogens. Teratogens can also increase the risk for miscarriage, preterm labor or stillbirth.

The following factors determine how dangerous teratogen exposure is during pregnancy:

  • The drug, substance or type of toxin.
  • How long the pregnant person was exposed.
  • The amount of exposure (dosage or quantity).
  • The gestational age of the fetus (weeks of pregnancy) at exposure.
  • Hereditary factors that could increase the fetus’s risk.

It’s best to avoid teratogens at all times during pregnancy. However, teratogenic exposure is more damaging at certain times in fetal development.


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How common are birth defects from teratogens?

Fetal exposure to teratogens accounts for about 4% to 5% of congenital disorders. Studies have also shown that exposure to teratogens affects cognitive and physical development.

What are examples of teratogens?

Teratogens are substances that can harm the fetus during pregnancy. Studies have shown that teratogens cause congenital disorders and increase the chance for miscarriage, stillbirth or other pregnancy complications.

Alcohol, cigarettes and recreational drugs

Alcohol, cigarettes and recreational drugs are known teratogens.

Alcohol affects the fetus’s central nervous system. Drinking alcohol during pregnancy increases the fetus’s risk for fetal alcohol syndrome. Fetal alcohol syndrome is a disorder that can cause abnormal facial features, a small head and brain and other physical and behavioral disabilities. There’s no amount of alcohol intake that’s considered safe during pregnancy.

Cigarette smoking is associated with fetal growth restriction, premature birth and miscarriage. Smoking also affects the fetus’s sensitive lung tissue and brain.

Using substances such as cocaine, methamphetamines, heroin and marijuana during pregnancy can cause low birth weight, heart problems and neonatal abstinence syndrome. This is when a baby goes through drug withdrawal after birth. Sharing needles can also cause infection. About 5% of people use these substances during pregnancy.


Certain over-the-counter (OTC) and prescription medications are considered teratogens. Therefore, it’s important to tell your healthcare provider about any medications you take. Read the labels before taking OTC medications or supplements. When in doubt about the safety of a substance, call your healthcare provider. It’s best to avoid the substance until you hear from them.

Examples of teratogenic medications are:

  • Antiepileptic drugs (AEDs).
  • Antimicrobials.
  • Anticoagulants (blood thinners).
  • Antithyroid medications.
  • Vitamin A (a common ingredient in skincare products).
  • Hormonal medication.

Healthcare providers weigh the pros and cons of prescription drug use to determine what poses less risk to the pregnancy. For example, phenytoin is a medication used for seizures. It has negative impacts on the fetus but may be medically necessary for the person who’s pregnant.

How is medication rated for safety during pregnancy?

The United States Food and Drug Administration (FDA) created five categories to classify the risks of medications during pregnancy. It consisted of categories A, B, C, D and X. While helpful, the category system oversimplifies a very complicated subject.

A newer system called the Pregnancy and Lactation Labeling Rule (PLLR) took effect in 2015. It removes the previous letter categories above. The PLLR gives healthcare providers relative information for decision-making when treating people who are pregnant or lactating. It considers many other factors and puts data in a better context for providers.

It’s best to let your pregnancy care provider decide on the safety of medication based on their expertise.

Infections and viruses

Infections, viruses, parasites and other bacterial illnesses can pose serious threats to a pregnant person and the fetus. The acronym TORCH helps to classify some of these:

Other infections and viruses that can cause pregnancy complications or problems with the fetus are:

Environmental toxins, chemicals or other physical agents

Certain chemicals and substances may cause congenital abnormalities. These birth disorders include spina bifida, cleft palate or neurological problems. Some examples of toxins or chemicals are:

  • Radiation exposure (from X-rays) or chemotherapy.
  • Hot tubs, saunas or other heat sources that raise your body temperature.
  • Mercury (found in certain types of fish).
  • Lead (commonly found in paint and pipes in older homes).
  • Toxic chemicals or heavy metals found in the workplace or manufacturing facilities.

Health conditions

Certain chronic illnesses can increase the fetus’s risk of congenital abnormalities. Some examples are diabetes, thyroid conditions and autoimmune diseases like lupus or rheumatoid arthritis. The increased risk for teratogenic exposure typically comes from the medication used to treat these conditions or these conditions not being well-controlled.

Talk to your healthcare provider about any preexisting conditions you have and what medications you take for treatment. You may need to adjust how your chronic medical condition is managed during your pregnancy.


When is teratogen exposure the worst during pregnancy?

Exposure to teratogens is harmful no matter what point you’re at in pregnancy. However, the risk is slightly higher during the first eight weeks of pregnancy. This is because many organs and systems are developing, making the fetus more sensitive to the harmful effects of teratogens. Studies show that teratogens can impact the fetus as early as two weeks from conception (when the sperm fertilizes an egg).

For example, neural tube defects (NTDs) occur before five weeks of pregnancy. The neural tube forms the brain and spine. NTDs occur when the neural tube doesn’t close properly. NTDs can cause several congenital disorders.

What birth defects do teratogens cause?

Teratogens cause many known congenital disorders. Some of the most common abnormalities are:


How can you avoid teratogens during pregnancy?

The best way to avoid teratogens is to plan for pregnancy, if possible. Planning for pregnancy allows you to get chronic medical conditions under control and make lifestyle changes such as quitting smoking.

However, this isn’t always possible. Once you’re pregnant, some things you can do to lower your risk of teratogen exposure are:

  • Talk with your healthcare provider about any medications you’re taking.
  • Avoid cigarettes, alcohol and recreational drugs.
  • Don’t take any supplements, medications or prescription drugs without checking with your healthcare provider.
  • Avoid cleaning litter boxes.
  • Avoid hot tubs, saunas and anything that raises your internal body temperature.
  • Remove tuna, swordfish and other fish high in mercury from your diet.
  • Talk with your supervisor or human resources about harmful chemicals in your workplace.

It’s important to have open and honest conversations with your obstetrician during pregnancy. This includes being truthful about alcohol or drug use. They’re there to make sure your pregnancy is safe and healthy. Don’t be afraid to contact your provider before taking medicines or supplements. It’s better to be overly cautious during pregnancy.

A note from Cleveland Clinic

Keeping the fetus safe from teratogens during pregnancy can help prevent congenital disabilities. The first step is to be aware that certain harmful substances can reach the fetus in the womb and negatively affect their development. Avoiding teratogens helps support a healthy pregnancy and gives your baby a great start in life. Next, be open with your healthcare provider about medications you take, as well as alcohol consumption and workplace or living conditions. They can help answer your questions about what substances may cause birth abnormalities and ensure you and the fetus are safe.

Care at Cleveland Clinic
Medically Reviewed

Last reviewed on 10/21/2022.

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