What Is a Teratogen?

A teratogen is a substance that may lead to birth defects in an embryo or fetus. During pregnancy, exposure to certain chemicals, infections, and drugs may increase the risk that a person will miscarry or that the embryo or fetus could have a developmental abnormality.

Alcohol and smoking are two common teratogens. Exposure to either of them can lead to developmental anomalies, miscarriage, stillbirth, preterm labor, and a variety of other pregnancy complications.

Types

The impact of teratogens on pregnancy or a fetus depends on several factors. The timing and length of exposure, the stage of pregnancy when the exposure happened, whether a parent’s genes make them more susceptible, and the type of agent they were exposed to all contribute to the risk. 

Teratogens generally fall under the following categories:

Drugs

Many pregnant people take prescription medications and over-the-counter (OTC) medications. Not all medications pose the same hazards, but it’s important to be aware of the medications that are known teratogens. 

• Certain prescription medications like some antiepileptic drugs (AEDs), antimicrobial medications, anticoagulants, vitamin A in large doses, and hormonal medications
• Some OTC medications: Read labels to learn about risks to pregnant people, and talk to your healthcare provider about any medications you are considering taking.
• Recreational drugs such as cocaine, marijuana, amphetamines, ecstasy, and heroin
• Alcohol
• Tobacco

Infections

Not all infections affect a developing fetus and pregnancy in the same way. Pregnant people often encounter the common cold and some milder infections without incident. But some illnesses have a greater potential to impact a pregnancy.

TORCH is an acronym that can help you remember the most common teratogenic viruses, parasites, and bacteria. They are:

• Toxoplasmosis: A parasitic infection most often associated with kitty litter
• Others: Like Treponema pallidum, a bacterial infection that causes syphilis
• Rubella: A viral infection that is also known as German measles
• Cytomegalovirus (CMV): A common viral infection
• Herpes simplex virus: A virus that presents as sores on or in the mouth (HSV1), or sores in or around the genitals (HSV2)

In addition to TORCH, some other infections that are teratogenic include:

• Varicella: Also called chickenpox
• Parvovirus B19: More commonly known as fifth disease
• Plasmodium falciparum: A parasite that causes malaria
• Listeria: A bacterial infection from eating contaminated food

Physical Agents

Exposure to certain necessary medical interventions, workplace hazards, or heat can pose a risk to fetuses as well. Radiation exposure and heat are both considered physical teratogens.

Radiation, also called electric and magnetic fields, is either ionizing or non-ionizing. Examples of non-ionizing radiation are microwaves, ultrasound, radio frequencies, computers, cell phones, and power lines. Studies have not found that these pose a significant risk during pregnancy.

Ionizing radiation includes things like X-rays, gamma rays, and sunlight. These can be harmful. The risk of exposure to ionizing radiation varies, depending on the dose of exposure and pregnancy stage. The highest risk is associated with higher doses and exposure in the first trimester.

Hyperthermia (abnormally high body temperature) can be teratogenic during pregnancy. Hyperthermia can be caused by things like exercise, hot tubs, and saunas. 

Mitigating hyperthermia risk involves ensuring that your core body temperature does not rise above 39 C (102.2 F). Research has found moderate exercise (defined as up to 35 minutes at 50% to 70% of maximum heart rate) and limited exposure to hot baths and saunas to be safe.

Environmental Toxins

Toxic metals and chemicals can pose risks to a developing fetus. Some examples of environmental toxins include:

• Mercury
• Lead
• Polychlorinated and polybrominated biphenyls (PCBs)

Mercury exposure most commonly happens through eating contaminated fish and getting amalgam (mercury) dental fillings. If you need dental work done while pregnant, talk to your dentist about using non-mercury filling material. 

Limit mercury exposure by avoiding certain fish that have high concentrations of mercury. King mackerel, marlin, orange roughy, shark, swordfish, tilefish, tuna, and bigeye are all fish that are highest in mercury.

The “Dietary Guidelines for Americans 2020–2025” recommends that pregnant people eat 8 to 12 ounces of seafood per week from choices that are low in methylmercury.

Lead exposure can happen from contact with lead in the air, soil, water, and household dust. Paint used in homes before 1978 often contained lead, and many older water pipes are made of lead.

You can limit your exposure to household lead by keeping painted surfaces in good shape, cleaning lead dust with a wet paper towel, using only certified lead professionals for renovations, wiping or removing shoes before entering your home, and having lead water lines replaced.

PCBs are found in fluorescent lights, fish caught in contaminated waters, older TVs and appliances, and soil around contaminated waste sites. You can limit your exposure by not eating fish caught in contaminated water, updating older appliances, and staying out of the soil near contaminated waste sites.

Maternal Health Conditions

Some maternal health conditions pose teratogenic risks. Chronic health conditions that can impact a developing fetus include:

• Uncontrolled diabetes
• Autoimmune disease
• Maternal phenylketonuria (PKU)

Research suggests that maternal autoimmune disease may be associated with some birth defects, but not with most of them. Risks associated with diabetes include birth defects of the spine, brain, limbs, and heart.

PKU is a genetic disorder that is passed down from parents to children. It is managed through a special diet. Unmanaged during pregnancy, it can lead to low birth weight, intellectual disabilities, heart defects, and other birth defects.

If you have a pre-existing health condition, it is very important to maintain treatment to limit the teratogenic risk during pregnancy. Talk to your healthcare provider about concerns you have about your health conditions and treatment options.

Exposure and Prevalence

About 4% to 5% of birth defects are caused by exposure to a teratogen. The risk of teratogens to a developing fetus is often early in the pregnancy—usually the first trimester and often before implantation. Of course, the impact also depends on the level of exposure. 

If a teratogen has the ability to affect the development of the neural tube, the exposure would need to occur before the time the neural tube closes, which is around three to four weeks. Some organ systems are susceptible throughout pregnancy, though.

The central nervous system, for example, can be impacted throughout an entire pregnancy. Alcohol is an example of a teratogen that can impact a developing fetus at any time during pregnancy.

Prevention

Sometimes it can be difficult to avoid teratogens altogether, especially if you have a pre-existing health condition or require medication. But, there are some things you can do to limit your exposure:

• Avoid alcohol, recreational drugs, and cigarettes while pregnant.
• Use good hand hygiene to avoid illnesses.
• Limit or avoid hot tubs and saunas.
• Manage any pre-existing health conditions.
• Discuss medications and vaccinations with your healthcare professional.

A Word From Verywell

It can be scary to think about bad outcomes and birth defects while you are pregnant. The good news is that exposure to teratogens only accounts for a very small percentage of birth defects. Most of the time, people are able to avoid the things that can negatively impact fetal development.

Even if you can’t avoid teratogens completely, there are things you can do to limit the risk to your developing fetus. Having proactive and open communication with your healthcare provider can help you manage any ongoing medical issues in a way that is safer for your pregnancy and your fetus.

Most of the time, people with health conditions and those who require medication go on to have full-term pregnancies and healthy babies.