Pregnancy and Medications: What You Need to Know About Teratogenic Risks and Birth Defects

When you’re pregnant, every pill, supplement, or over-the-counter remedy feels like a potential gamble. You want to feel better-maybe you have a headache, nausea, or anxiety-but you’re terrified that the very thing helping you could harm your baby. This isn’t just fear; it’s a real, science-backed concern. About 2-3% of all birth defects are linked to medications taken during pregnancy. That number might sound small, but for the families affected, it’s everything.

What Exactly Is a Teratogen?

A teratogen is any substance that can interfere with fetal development and cause birth defects. It’s not just illegal drugs or dangerous chemicals-it includes prescription meds, over-the-counter painkillers, and even some herbal products. The most infamous example is thalidomide. In the late 1950s, this drug was prescribed to pregnant women for morning sickness. By 1961, more than 10,000 babies worldwide were born with missing or shortened limbs. That tragedy changed medicine forever.

Today, we know teratogens don’t just cause obvious physical deformities. They can lead to heart defects, brain damage, vision or hearing loss, and even long-term learning disabilities. The timing of exposure matters more than you think. The first 12 weeks of pregnancy-when organs are forming-are the most dangerous window. Between days 15 and 60 after conception, the embryo is especially sensitive. A medication that’s harmless in week 14 could have been devastating in week 6.

Medications with Proven Risks

Not all drugs are equally risky. Some have clear, well-documented dangers. Here are a few that doctors strongly advise against during pregnancy:

• Warfarin: This blood thinner can cause fetal warfarin syndrome-characterized by a flattened nose, bone problems, eye damage, and developmental delays. The risk is highest in the first trimester, but even later use can lead to bleeding in the baby or brain damage.
• Methotrexate: Used for autoimmune diseases and cancer, this drug blocks folate, a nutrient critical for spinal cord development. Exposure in early pregnancy raises the risk of neural tube defects by 10-20%.
• Carbamazepine: An anti-seizure medication that increases the chance of spina bifida by about 1%. It can also cause bleeding in newborns due to low vitamin K.
• Factor Xa inhibitors (rivaroxaban, apixaban, edoxaban): These newer blood thinners cross the placenta, and there’s no way to reverse their effects if bleeding occurs. No safe dose has been established.
• Cannabis (THC): Even if you think marijuana is "natural," THC crosses the placenta. Studies show a 15-20% higher risk of low birth weight, 10-15% higher risk of preterm birth, and possible long-term effects like attention problems and lower IQ scores in children.

Some medications, like isotretinoin (Accutane) for acne, are so dangerous they come with mandatory pregnancy tests and counseling before prescription. If you’re on any of these, talk to your doctor immediately-do not stop cold turkey without guidance.

The Acetaminophen Controversy

No drug in pregnancy has sparked more debate than acetaminophen (Tylenol). For decades, it was considered the safest option for fever and pain. But recent studies have raised red flags. Some research links long-term, high-dose use during pregnancy to a slightly higher risk of ADHD and autism in children.

Here’s the catch: no one has proven acetaminophen causes these conditions. The CDC says the evidence is "suggestive," while the American College of Obstetricians and Gynecologists (ACOG) issued a strong statement in September 2025 saying acetaminophen remains the best and safest choice for pain and fever in pregnancy. Why? Because untreated fever-especially above 101°F-can raise the risk of neural tube defects by 20-30%.

So what’s a pregnant person supposed to do? ACOG’s message is clear: the risks of not treating pain or fever are far greater than the theoretical risks of acetaminophen. Use the lowest effective dose for the shortest time. Don’t panic, but don’t take it daily without a reason.

Timing Matters: First, Second, Third Trimester

The effects of medications change depending on when you take them:

• First trimester (weeks 1-12): This is when organs form. Exposure here can cause major structural defects-heart, brain, limbs, face.
• Second trimester (weeks 13-26): Major organs are done forming, but the brain and genitals are still developing. Risks shift toward functional problems-like hearing loss, behavior changes, or subtle growth delays.
• Third trimester (weeks 27-birth): The baby’s organs are mostly built, but they’re still maturing. Medications here might cause withdrawal symptoms (like with opioids), low blood sugar, or breathing problems after birth. Some drugs can also trigger early labor.

Even if you took a medication before you knew you were pregnant, don’t panic. Most exposures don’t lead to problems. But do tell your provider right away. They can help you assess the risk based on timing, dosage, and your medical history.

How Medications Actually Affect the Fetus

It’s not just about the drug itself. Five main pathways determine how a medication impacts pregnancy:

1. No effect: About 60-70% of medications pass through the placenta with no harm to the fetus.
2. Direct damage: The drug attacks developing cells-like methotrexate disrupting DNA.
3. Indirect harm: A drug lowers your blood pressure so much that less oxygen reaches the baby.
4. Placental interference: Some meds block nutrients or oxygen from crossing into the placenta.
5. Uterine stimulation: Certain drugs can trigger contractions, leading to preterm birth.

This is why a drug that’s safe for a non-pregnant person might be dangerous during pregnancy. It’s not just about toxicity-it’s about how the body changes during pregnancy and how the baby responds.

What You Should Do Before and During Pregnancy

You don’t have to guess. Here’s how to take control:

• Before you get pregnant: Review all your medications with your doctor or pharmacist. If you’re on something risky, like an anti-seizure drug or antidepressant, there may be safer alternatives.
• During pregnancy: Never start, stop, or change a medication without talking to your provider. Even "natural" supplements like St. John’s Wort or high-dose vitamin A can be dangerous.
• Use trusted resources: Don’t rely on Google or Reddit. Use MotherToBaby (a free service run by teratology experts), LactMed (for breastfeeding safety), or the FDA’s pregnancy labeling.
• Ask questions: "Is this necessary?" "Is there a safer option?" "What happens if I don’t take it?" These are valid, important questions.

Here’s something many women don’t realize: about 40-50% of pregnancies are unplanned. If you’re sexually active and not using birth control, assume you could be pregnant. That means avoiding known teratogens like isotretinoin, ACE inhibitors, and high-dose NSAIDs even before you test positive.

The Reality of Missing Data

Here’s the uncomfortable truth: for about 70-80% of medications, we simply don’t have enough data on safety during pregnancy. Why? Because we can’t ethically test drugs on pregnant women in clinical trials. Most of what we know comes from animal studies, accidental exposures, or tracking women who took medications before realizing they were pregnant.

That’s why pregnancy registries exist-voluntary programs where women report what they took and how their babies turned out. But only 15-20% of eligible women join them. The FDA is working on using real-world data from millions of electronic health records to fill these gaps, but it’s slow.

Until then, the safest approach is this: if you don’t need it, don’t take it. If you do need it, use the lowest dose for the shortest time-and always get professional advice.

What About Mental Health Medications?

This is one of the hardest decisions. Untreated depression or anxiety during pregnancy can lead to poor nutrition, preterm birth, low birth weight, and even postpartum depression that affects bonding and infant development.

Some antidepressants, like SSRIs (sertraline, citalopram), have been studied more than others. While there’s a small increased risk of heart defects or persistent pulmonary hypertension in newborns, the overall risk remains low. Stopping medication abruptly can trigger relapse-which is often more dangerous than the drug itself.

ACOG and the American Psychiatric Association agree: the decision must be individualized. For some women, staying on medication is the safest choice. For others, therapy or lifestyle changes might work better. Don’t let fear silence your needs.

Final Takeaway: Balance, Not Fear

Pregnancy isn’t the time to panic about every pill. It’s the time to be informed. Most medications don’t cause birth defects. But some do-and the stakes are high enough that you need to be proactive.

The goal isn’t to avoid all drugs. It’s to avoid unnecessary ones. Use what you need, when you need it, at the lowest dose possible. Talk to your provider. Use reliable sources. And remember: untreated illness can be just as dangerous as medication.