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 gamble. You want to feel better-maybe you have a headache, nausea, or anxiety-but you’re terrified that the medicine might hurt your baby. You’re not alone. About 90% of pregnant women take at least one medication during pregnancy, and nearly half of all pregnancies are unplanned, meaning many women are already taking drugs before they even know they’re pregnant. The real question isn’t whether to take medicine-it’s which ones are safe, when, and why.

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 medications, over-the-counter painkillers, and even some herbal products. The most infamous example is thalidomide, a sedative given to pregnant women in the 1950s for morning sickness. By the time it was pulled from the market in 1961, it had caused severe limb deformities in about 10,000 babies worldwide. That tragedy changed how we test drugs for pregnancy safety forever.

Today, we know that about 2-3% of all birth defects are linked to medications. That sounds low, but it’s still thousands of babies each year. The risk isn’t the same for every drug or every stage of pregnancy. The most dangerous window is between weeks 3 and 8 after conception-when the baby’s organs are forming. After that, the risks shift from structural defects to problems with brain development, growth, or organ function.

Medications with Proven Teratogenic Risks

Some drugs are known to be dangerous during pregnancy, and doctors avoid them unless there’s no other option. Here are a few with strong evidence:

• Warfarin (a blood thinner): Can cause fetal warfarin syndrome-nose underdevelopment, bone abnormalities, vision loss, and intellectual disability. Risk is highest in the first trimester.
• Methotrexate (used for autoimmune diseases and cancer): Increases neural tube defect risk by 10-20%. Must be stopped at least 3 months before trying to conceive.
• Carbamazepine (for epilepsy): Raises risk of spina bifida by about 1%. Also causes vitamin K deficiency in newborns, leading to dangerous bleeding.
• Factor Xa inhibitors (like rivaroxaban, apixaban): These newer blood thinners cross the placenta and have no antidote. No safe dose has been established.
• Cannabis (THC): Linked to lower birth weight, preterm birth, and stillbirth. THC stays in breastmilk for up to six days and may affect baby’s brain development.

These aren’t hypothetical risks. They’re documented in clinical studies, case reports, and national registries. If you’re taking any of these, talk to your doctor before getting pregnant-or as soon as you find out you’re pregnant.

The Acetaminophen Controversy

Acetaminophen (Tylenol) is the most commonly used pain reliever during pregnancy. For decades, it was considered the safest option. But in recent years, some studies have linked long-term use to higher chances of ADHD and autism in children. The CDC says these associations exist, but they haven’t proven cause and effect.

Meanwhile, the American College of Obstetricians and Gynecologists (ACOG) issued a clear statement in September 2025: "The conditions people use acetaminophen to treat during pregnancy are far more dangerous than any theoretical risks." Untreated fever, for example, can increase the risk of neural tube defects by 20-30%. Severe pain or chronic headaches can lead to poor nutrition, stress, and even preterm labor.

So what’s the real advice? Use acetaminophen only when needed, at the lowest effective dose, and for the shortest time possible. Don’t panic if you took it early in pregnancy-most babies are fine. But don’t treat it like candy either. If you’re taking it daily for weeks, talk to your provider about alternatives or underlying causes.

How Pregnancy Stages Affect Drug Risks

Not all drugs are dangerous all the time. Timing matters more than you think.

• Weeks 1-12 (First Trimester): This is the most sensitive period. The baby’s heart, brain, limbs, and organs are forming. Exposure to teratogens here can cause major structural defects like cleft palate, heart problems, or missing limbs.
• Weeks 13-26 (Second Trimester): Major organs are done forming, but the brain and genitals are still developing. Risks shift to functional problems-like hearing loss, learning delays, or genital abnormalities.
• Weeks 27-Birth (Third Trimester): Structural defects are rare, but drugs can still cause issues like withdrawal symptoms (if mom is on opioids), low blood sugar, or altered brain development. Some medications can also trigger early labor.

That’s why stopping a medication cold turkey in the first trimester can be more dangerous than continuing it. For example, if you have epilepsy and stop your seizure medicine, you could have a seizure-and that’s far riskier for the baby than the drug itself.

Why So Much Uncertainty?

You’ve probably read conflicting advice online. One site says Zofran is safe for nausea. Another says it might cause cleft palate. Why the confusion?

Because we can’t do controlled experiments on pregnant women. It’s unethical. So almost all our data comes from:

• Observational studies (tracking women who took drugs by accident or necessity)
• Animal studies (which don’t always match human biology)
• Case reports (single stories, not proof)

That means we’re working with incomplete data. About 70-80% of medications have no solid safety data in pregnancy. Even common ones like antidepressants or allergy meds are often based on limited evidence.

That’s why services like MotherToBaby exist. They handle over 10,000 calls a year from worried parents, mostly about acetaminophen, antidepressants, and nausea meds. Their advice? Don’t guess. Call a specialist.

What Should You Do?

Here’s a practical plan:

1. If you’re planning pregnancy: Review every medication you take-with your doctor and pharmacist. Include vitamins, supplements, and herbal teas. Some, like high-dose vitamin A or St. John’s Wort, can be harmful.
2. If you’re already pregnant: Don’t stop anything without talking to your provider. Many women panic and quit their meds, then end up worse off. High blood pressure, untreated depression, or uncontrolled asthma can harm your baby more than the drug.
3. Use reliable sources: Skip Google. Use MotherToBaby, LactMed (from the NIH), or the FDA’s Pregnancy and Lactation Labeling Rule (PLLR) information on prescription labels. The old A, B, C, D, X categories are outdated. New labels give you real-world context: "Risk in first trimester: increased risk of neural tube defects based on 3 cohort studies".
4. Ask for help: Your OB-GYN may not be a drug expert. Ask if you can consult a maternal-fetal medicine specialist or clinical pharmacist. Many hospitals have them on staff.

What’s Changing in 2025 and Beyond

There’s hope on the horizon. The FDA’s Sentinel Initiative is tracking 10 million patient records to find patterns in real-world pregnancy outcomes. By 2026, we’ll have better data on commonly used drugs.

Experts like Dr. Christina Chambers predict that within five years, we’ll use pharmacogenomics-testing your genes-to predict how you and your baby will respond to certain drugs. This could cut risks by 30-40%.

But progress is slow. Only 2-3% of maternal health research funding goes to medication safety. And only 15-20% of eligible women join pregnancy registries that track drug exposure. We need more participation to get better answers.

Final Thoughts

Pregnancy isn’t the time to play Russian roulette with medications. But it’s also not the time to go without treatment because you’re scared. The goal isn’t zero risk-it’s informed risk. Most medications don’t cause birth defects. But some do, and the consequences can be life-changing.

Take charge: Know what you’re taking. Ask questions. Use trusted sources. And remember-you’re not alone. Millions of women have faced this exact fear. The right choice isn’t always obvious, but with the right information, you can make a decision that protects both you and your baby.