Teratogens in Pregnancy: Effects on Fetal Development

A teratogen is anything that causes congenital abnormalities in a developing fetus. The term includes chemicals and substances, but also environmental conditions, infections, and illnesses (Healthline, 2023). Teratogens interfere with fetal development, cause birth defects, and can cause lifelong health problems and disabilities.

What are some teratogens to which pregnant people are most commonly exposed? What are some of the effects that they cause, and how can one avoid them?

What are Examples of Teratogens in Pregnancy?

There are a lot of things that can harm your growing baby. Some are more obvious than others.

Alcohol

In addition to risks to your own health, alcohol, tobacco, and recreational drugs pose serious hazards to a developing fetus.

The best-known effect of prenatal exposure to alcohol is Fetal Alcohol Syndrome, also called Fetal Alcohol Spectrum Disorder (FASD). FASD can cause lifelong health problems (Healthline, 2017), including:

• Deformed limbs, fingers, or facial features

• Hyperactivity
• Intellectual delays
• Speech delays
• Poor judgment
• Learning disabilities
• Heart and kidney defects
• And more

Alcohol use by pregnant women is also a risk factor for low birth weight, preterm birth, and stillbirth.

Tobacco

Smoking and vaping tobacco expose a baby to the known teratogen nicotine. Nicotine puts a baby at greater risk for Sudden Infant Death Syndrome (SIDS), as well as:

• abnormal brain development
• damage to the placenta
• abnormal heart development
• immune system abnormalities
• low birth weight
• premature birth
• thyroid problems
• elevated blood sugar

• chronic ear infections
• lower respiratory infections like bronchitis and pneumonia
• abnormal lung function
• other serious birth defects

Recreational Drugs

The problems that recreational drugs can cause vary according to the drug. In addition to increasing the risk of miscarriage, preterm birt, and stillbirths, recreational drug use during pregnancy can result in:

• Cognitive performance deficits
• Information-processing problems
• Attention deficits
• Urinary tract defects
• Heart defects
• Lower IQ
• In-vitro stroke
• Brain damage
• And more (Bruce, 2021).

So if you’re pregnant or trying to get pregnant, don’t drink, smoke — including vapes! — or use recreational drugs.

Caffeine

Although the effects of caffeine on a developing fetus aren’t as dramatic as those of tobacco and alcohol, excessive amounts of caffeine can cause your baby to have a temporary irregular heartbeat, respiration, and tremors. In addition, large quantities of caffeine can cause low birth weight, which, in turn, can cause problems for your newborn.

Medications That Are Known Teratogens 

The following classes of medications are known teratogens (Tsamantioti, 2022). This is by no means a complete list of teratogenic medications. If you have questions about how a medication you’re taking may affect your developing baby, it’s important to speak to your healthcare provider.

Medications for Neurological Conditions

Medications for neurological conditions include medicines taken for psychiatric conditions, epilepsy, migraines, and other conditions. There are many medicines in this category. Some of the more common ones include Valproate, Phenobarbital, and Carbamazepine.

Valproate (Depakote) can lead to cardiac anomalies, neural tube defects, spina bifida, and developmental delay. Less commonly it can cause Fetal Valproate Syndrome, which includes congenital malformations of limbs, cleft lip and/or cleft palate, and urinary tract defects.

Phenobarbital increases the risk for impaired growth, impaired motor development, and fetal death.

Carbamazepine (Tegretol) use during pregnancy can result in craniofacial defects, abnormal IQ, and growth retardation.

Antimicrobial Medications

Two antimicrobial medications that should definitely be avoided during pregnancy include fluoroquinolones and tetracyclines. Tetracyclines can suppress bone growth, as well as result in liver necrosis, bone, and teeth defects. The use of fluoroquinolones can result in renal, cardiac, and central nervous system toxicity, and other serious problems.

Anticoagulants

Anticoagulant medication like Warfarin can cause skeletal abnormalities, such as nasal hypoplasia, which can, in turn, cause feeding and breathing difficulties.

Over-the-Counter Medications, Herbs, and Supplements

Just because you can buy something without a prescription doesn’t mean it can’t hurt your baby.

Acetaminophen (Tylenol) is many physicians’ analgesic of choice and is relatively safe during pregnancy. Ibuprofen (Motrin), Aspirin, and Naproxen (Aleve), on the other hand, should not be used during pregnancy, especially during the third trimester (Servey, 2014).

Vitamin A in large amounts has been shown to cause spina bifida, hydrocephalus, neural tube defects, eye malformations and cleft palate in laboratory animals. It can also cause heart defects.

Black Cohosh and Dong Quai can cause uterine contractions, which could induce preterm labor. Black Cohosh can also cause liver toxicity (Kubala, 2020).

Other herbs to be avoided during pregnancy include (but are not limited to): saw palmetto, tansy, red clover, angelica, yarrow, wormwood, blue cohosh, goldenseal, pennyroyal, ephedra, and mugwort (Kubala, 2020).

Be sure to discuss any over-the-counter medications or supplements you may be taking with your healthcare provider.

Exposure to Chemicals and Toxins in Pregnancy

Exposure to certain chemicals and toxins can harm you, your baby, or both. Some of the more commonly encountered chemicals and toxins include iodine, lead, mercury, and carbon monoxide.

Effects of Infections and Viruses During Pregnancy

In addition to physical agents, some infections, viruses, and diseases can also be teratogenic agents.

Toxoplasmosis, which is caused by a parasite in cat feces, can be transmitted from mother to child, potentially causing brain and eye damage to the child.

Infection with Rubella (German Measles) can cause cataracts, deafness, and problems with the brain and heart.

The teratogenic effects of chickenpox range from mild to severe, and can include:

• Brain damage (encephalitis, microcephaly, hydrocephaly, brain aplasia)
• Deformities and damage to the eye
• Neurological and spinal cord disorders
• Motor and sensory deficits
• Bladder and anal sphincter dysfunction
• Skin lesions and disorders
• And more

Other health conditions that can affect your baby include Hepatitis B and C, HIV, syphilis and cytomegalovirus (a herpes simplex infection).

Risks of Physical and Environmental Teratogens 

Excessive heat from fever, hot tubs, saunas, or exposure to extremely hot weather can cause low birth weight, preterm birth, stillbirth, and other problems (Syed, 2022).

Indoor and outdoor air pollution can have a range of negative effects on pregnant women, developing babies and children.

Maternal Health Conditions and Fetal Risks 

A mother’s health conditions can also harm a growing baby. If you have any of these pre-existing health conditions, it’s important to consult closely with your healthcare provider throughout your pregnancy.

Diabetes and Gestational Diabetes can cause high blood sugar levels in a developing fetus. This has been linked to neural tube, brain, and spinal cord defects. 

Thyroid conditions, Lupus, and Rheumatoid Arthritis can also have teratogenic effects, whether from uncontrolled disease or from the medications used to control them.

At What Stage of Pregnancy Are Teratogens Most Harmful?

At what stage of gestation do teratogens cause the most damage? In the first trimester? Or later?

Although the first eight weeks of pregnancy are a particularly vulnerable time, many things can harm your baby at other points in the pregnancy, too.

Some teratogens, like alcohol and tobacco, can harm your baby even before the egg implants in the uterus. Others, like many NSAIDS, are more harmful in later pregnancy than earlier on.

Some substances harm specific parts of the baby as those parts are developing. This is true, for example, of substances that cause neural tube defects like spina bifida. Other substances can cause harm throughout the pregnancy.

What is the Teratogenic Window?

The teratogenic window is the period in early pregnancy when birth defects are most likely to occur. Some define this period as being generally between 30 days before conception and 14 weeks of gestation. The teratogenic window for many birth defects is during gestational weeks six to eight (Caton, 2012).

Symptoms of Teratogen Exposure in Your Baby

Because there are so many teratogens, and they can cause a wide array of birth defects, it can be difficult to pinpoint the exact cause of any given problem. If you suspect that your baby has a problem caused by a teratogenic agent, your doctor is your first stop for more information.

Reducing Exposure to Teratogens During Pregnancy

If you drink alcohol, smoke or vape, or use recreational drugs, stop. And if you’re a heavy caffeine user, reduce or eliminate your caffeine use.

Avoid people with visible signs of illness, such as runny noses or coughing, and wash your hands frequently.

Avoid going outside during periods of excessive heat, as well as when your area is under an air quality warning. Avoid hot tubs and saunas.

Speak to your doctor about any medications you may be taking that might have teratogenic effects. If any of them can harm your baby, your doctor may be able to recommend a safer alternative.