What Is a Teratogen?

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Teratogens are any agent that disrupts a baby's development when a person is exposed to it during pregnancy. Known teratogens include alcohol, smoking, toxic chemicals, radiation, viruses, some maternal health conditions, and certain prescription drugs. Some teratogens are more concerning than others. How harmful a teratogen is depends on several factors, including the type, amount, duration, and time of exposure as well as genetic influences.

  • Type: Some teratogens, including alcohol and smoking, have a more harmful impact on a developing fetus than others.
  • Amount: The amount of harm to a fetus increases the more the pregnant person consumes or is exposed to a teratogen.
  • Duration of exposure: The longer a fetus is exposed to a teratogen, the more dangerous the effects.
  • Time of exposure: Teratogens are most harmful early in pregnancy, starting about 10 to 14 days after conception to about 8 weeks into pregnancy.
  • Genetics: Sometimes, the pregnant person's or the baby's unique genetics protect them or make them more vulnerable to certain teratogens.

Why They Are Important

All parents should know what teratogens are and how to avoid them since they can cause harm throughout pregnancy, starting around the time of conception. For instance, the risk of miscarriage is higher when you smoke or drink alcohol or are exposed to radiation and certain toxic chemicals. These and other teratogens are also linked to premature birth.

Additionally, 4% to 5% of birth defects are caused by teratogens. These include physical malformations, like spina bifida, cleft palate, or microcephaly (when the brain and skull are underdeveloped), as well as vision and hearing problems. Teratogens can also affect cognitive development. Babies born to a parent who drinks alcohol or has certain health conditions, like thyroid disorders, have been shown to have lower IQ scores.

Teratogens can cause harm from conception to delivery but often cause the most damage in the first 8 weeks of pregnancy.

Types of Teratogens

Teratogens can lurk in your house, garden, medicine cabinet, and even inside your body. Knowing what these dangerous agents are is the first step to protecting yourself and your baby.


Wine, beer, and all other types of alcohol can harm a baby during pregnancy. Alcohol passes through the umbilical cord to a developing baby and can cause stillbirth and miscarriage. Alcohol can also create a lifelong spectrum of physical, behavioral, and intellectual disorders known as fetal alcohol syndrome.

Smoking Cigarettes

Smoking cigarettes negatively impacts fertility, increases the risk for pregnancy complications like stillbirth and miscarriage, and is linked to several birth defects. Specifically, smoking can damage fragile, growing tissue in a developing baby's lungs and brain.

The risks of using nicotine-containing e-cigarettes and vaping devices during pregnancy aren't fully known. However, early research suggests that exposure to nicotine in the womb can cause damage to babies' developing cardiovascular, immune, nervous, and respiratory systems, even in the absence of tobacco smoke.

Recreational Drugs

About 1 in 20 pregnant people takes street drugs like marijuana, cocaine, and heroin during pregnancy, which is very harmful to fetal development. These drugs are linked to infertility, premature birth, miscarriage, low birth weight, and birth defects.

Certain Medications

Even some doctor-approved medications can be harmful as soon as you conceive. Neurological drugs, like antiepileptic medications, can cause cognitive defects in babies, and blood thinners are linked to certain congenital malformations. Accutane, retinoids, and other skincare medications containing vitamin A can cause serious abnormalities, including cleft palates and intellectual disabilities.

Some Infections

A pregnant person's exposure to some viruses, bacteria, and parasites can pose harm to the baby. Some of the more dangerous infections are grouped under the acronym TORCH:

  • Toxoplasma (often passed through cat feces)
  • Rubella (German measles)
  • Cytomegalovirus
  • Herpes simplex virus

The risk of some infections is higher in certain geographic regions. Since 2015, when there was an outbreak of the mosquito-borne Zika virus throughout Central and South America and parts of the southern United States, 10% of babies born in the U.S. to mothers with an active Zika infection during pregnancy had birth defects.

Flu is not linked to birth defects, but the high fevers it can bring on are. Pregnant people with COVID-19 are at higher risk for health problems like preeclampsia and infections, and their babies are more likely to be born prematurely.

Harmful Physical Agents

Even in the womb, babies are sensitive to some invisible environmental forces. These include radiation, like from x-rays, or heat sources that cause your temperature to exceed 102 degrees Fahrenheit for a prolonged period, such as excessive hot tub use or a high fever.

Toxic Chemicals and Substances

Pregnant people are often inadvertently exposed to toxic chemicals in certain workplaces like industrial factories, dry-cleaning facilities, and medical offices. Some dangerous substances that are used in these workplaces include heavy metals, organic solvents, and petrochemicals.

Excessive exposure to household lead (like in old paint and construction) and dietary mercury (found in tuna and other big fish) are also known to cause neurological issues and cognitive delays.

Maternal Health Problems

You can have a physical condition yourself that can hamper fetal development, which is one reason your health should be closely monitored before and during pregnancy. For example, untreated diabetes can cause miscarriage, stillbirth, and fetal abnormalities. Babies born to parents with unchecked thyroid issues can have lower IQs and motor skill problems.


Anything that enters your blood stream and could hurt your unborn child in some way is considered a teratogen. To stop as many of these harmful substances as possible from reaching your baby, first look at anything you consume, such as food, medicine, or drinks. Next, remove or swap out any toxic chemicals used in cleaning, your workplace, or elsewhere. Ask your provider if you are concerned that you may be exposed to anything that could negatively affect your baby.

How to Avoid Teratogens

Some teratogens, like certain genetic conditions, are tough to avoid. But there are many things you can do to steer clear of some of the most dangerous teratogens. Because teratogens cause harm starting around conception, it's a good idea to take these steps while trying to get pregnant or first learning you are pregnant:

  • Avoid alcohol. There is no known safe amount of alcohol during pregnancy.
  • Avoid or quit cigarettes. Since quitting smoking can take some time and support, dropping the habit before becoming pregnant is best.
  • Develop scrupulous handwashing habits. Keeping your hands clean helps you avoid infections and potentially dangerous chemical residues.
  • Don't clean cat litter during pregnancy. This will reduce your risk of toxoplasmosis.
  • Rid your diet of unpasteurized dairy products and deli meats. They can harbor harmful bacteria.
  • Cut back on tuna and swordfish during pregnancy. Experts recommend eating no more than 6 ounces per week.
  • Use organic household cleaners and lawn care products. These reduce your risk of chemical exposures.
  • Don't take any medications without consulting with your doctor first. That includes acne treatments and over-the-counter pain relievers.
  • Inquire with your workplace's human resources department about any toxic chemicals you may be exposed to at work. A leave of absence to protect yourself may be warranted.
  • Seek out or maintain treatment for chronic health conditions. It's important to manage your own health and ensure that chronic conditions like diabetes or thyroid disorders are well managed before, during, and after pregnancy.

Regulation of Teratogens

The government has enacted legislation to raise awareness of and reduce the risks of certain teratogens. For example, thalidomide was a medication commonly given to pregnant people in the 1950s to treat morning sickness. It was later found to cause malformations in babies' limbs, organs, and other body parts. Banned in 1961, thalidomide was one of the first teratogens to be discovered and regulated to protect pregnant people and babies.

Since then, tobacco companies have been ordered to include a Surgeon General's warning about the dangers of smoking during pregnancy on each cigarette packet. Standards for the use of certain toxic chemicals that harm reproductive health, like lead, have been set by the Occupational Safety and Health Administration (OSHA).

A Word From Verywell

There is no way to shield your unborn child from every potential developmental concern. But knowing about and doing your best to avoid the most dangerous teratogens, like alcohol and smoking, is an important step to give your baby the best possible chance of growing healthy and strong.

Scientists are hard at work to identify new causes of pregnancy complications, birth defects, and developmental delays every day. So be sure to keep in close touch with a doctor about what you are consuming, where you are living and working, and where you might be traveling, from the moment you find out you're pregnant (if not before). You and your baby will benefit from your efforts to do all you can to support a healthy, happy pregnancy.

15 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Krissi Danielsson
Krissi Danielsson, MD is a doctor of family medicine and an advocate for those who have experienced miscarriage.