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 medicine feels like a gamble. You want to feel better - maybe you have a headache, nausea, or anxiety - but you’re terrified of hurting your baby. The truth is, teratogenic medications can cause real harm, but most drugs don’t. And the biggest risk isn’t always the medicine - it’s the untreated condition.

What Exactly Is a Teratogen?

A teratogen is any substance that can interfere with fetal development and lead to birth defects. It’s not just illegal drugs or experimental chemicals. Many common prescription and over-the-counter medicines fall into this category. The term comes from the Greek word teras, meaning monster - and that’s not an exaggeration. The thalidomide disaster in the 1950s and 60s showed the world what happens when a drug crosses the placenta unchecked. Around 10,000 babies were born with missing or shortened limbs after their mothers took it for morning sickness. That tragedy changed medicine forever.

Today, we know that about 2-3% of all birth defects are linked to medications taken during pregnancy. That sounds low, but it’s still thousands of families affected every year. The key isn’t avoiding all drugs - it’s knowing which ones to avoid, when, and why.

When Is the Baby Most at Risk?

Not all stages of pregnancy carry the same level of risk. The first trimester - especially weeks 3 to 8 after conception - is when the baby’s organs are forming. That’s the window when teratogens are most likely to cause major structural defects like heart problems, cleft lip, or missing limbs. After week 12, the risk of major physical deformities drops, but damage to brain development, hearing, vision, or organ function can still happen.

In the second trimester, medications are more likely to affect growth or cause minor abnormalities. By the third trimester, the baby’s body is mostly built - but the brain is still wiring itself. That’s when drugs can cause problems like withdrawal symptoms, breathing issues, or long-term behavioral changes. Even a short course of certain antibiotics or painkillers late in pregnancy can have consequences.

Medications with Proven Teratogenic Risks

Some drugs are known to be dangerous at any stage of pregnancy. Here are the ones you need to know:

• Warfarin - Used to prevent blood clots, but it can cause fetal warfarin syndrome: underdeveloped nose, bone problems, eye damage, and intellectual disability. Risk is highest in the first trimester.
• Methotrexate - A drug for cancer and autoimmune diseases. It blocks folate, which is essential for neural tube development. Exposure in early pregnancy increases the risk of spina bifida and other defects by 10-20%.
• Carbamazepine - An anti-seizure medication linked to a 1% risk of neural tube defects. It also lowers vitamin K, which can cause dangerous bleeding in newborns.
• Factor Xa inhibitors - Drugs like rivaroxaban and apixaban. We don’t have enough human data, but animal studies show they cross the placenta. There’s no antidote if something goes wrong.
• Cannabis (THC) - Even if you think it’s “natural,” THC crosses the placenta. Studies show a 15-20% higher chance of low birth weight, 10-15% higher risk of preterm birth, and possible long-term effects on attention and learning. THC stays in breastmilk for up to six days.

These aren’t hypothetical risks. These are documented, repeated findings from thousands of pregnancies tracked over decades.

The Acetaminophen Controversy

This is where things get messy. Acetaminophen - Tylenol, paracetamol - is the go-to pain reliever for pregnant women. It’s been used for generations. But in recent years, some studies started linking long-term use to higher rates of ADHD and autism in children. The CDC flagged this as a potential concern. Then, in September 2025, the American College of Obstetricians and Gynecologists (ACOG) pushed back hard.

ACOG said: “The conditions you treat with acetaminophen - fever, pain - are far more dangerous than any theoretical risk.” Untreated fever during pregnancy increases the risk of neural tube defects by 20-30%. Chronic pain can lead to depression, poor nutrition, and preterm labor. If you avoid acetaminophen because you’re scared, you might be putting your baby at greater risk.

The truth? There’s no clear proof that acetaminophen causes autism or ADHD. But there’s strong proof that uncontrolled pain and fever hurt babies. So ACOG’s advice is simple: use it when needed, at the lowest dose for the shortest time. Don’t avoid it out of fear. Don’t overuse it out of convenience.

How Medications Harm the Fetus

Not all damage comes from direct toxicity. Here are five ways drugs can affect your baby:

1. No effect - About 60-70% of medications don’t cross the placenta or don’t harm the baby. Many antibiotics, antacids, and allergy meds fall here.
2. Direct damage - The drug attacks developing cells. Like methotrexate blocking folate.
3. Indirect damage - The drug lowers your blood pressure so much that less oxygen and nutrients reach the placenta.
4. Placental interference - Some drugs change how the placenta works, reducing nutrient flow.
5. Uterine stimulation - Certain medications can trigger contractions, leading to preterm birth.

That’s why it’s not enough to just check a drug’s name. You need to understand how it works in your body - and how that affects your baby.

What You Should Do Before and During Pregnancy

You don’t need to panic. But you do need to be proactive.

• Before you get pregnant - Talk to your doctor about every medication you take. That includes supplements, herbal remedies, and acne treatments. Some birth control pills, antidepressants, and blood pressure meds need to be switched out before conception.
• If you’re already pregnant - Don’t stop any medication without talking to your provider. Stopping seizure meds, insulin, or antidepressants can be more dangerous than keeping them.
• Use reliable sources - Don’t trust random blogs or Reddit threads. Use MotherToBaby, LactMed, or your pharmacist. These are science-backed, free services staffed by specialists who handle over 10,000 pregnancy medication questions every year.
• Check labels - The FDA no longer uses the old A, B, C, D, X categories. Now, prescription labels have detailed sections on pregnancy risks, clinical considerations, and data sources. Read them.

Here’s the hard truth: about 90% of pregnant women take at least one medication. Most are safe. But you can’t assume. You need to ask.

Why So Little Is Known

You might wonder: why don’t we know more? Why are so many drugs still a mystery?

Because we can’t test them properly. Ethical rules prevent giving pregnant women new drugs in clinical trials just to see what happens. So 95% of what we know comes from watching what happens after the fact - case reports, registries, and observational studies. That’s why the data is messy. That’s why experts disagree.

The FDA is trying to fix this with its Sentinel Initiative, tracking 10 million patient records to find patterns. But progress is slow. Right now, 70-80% of medications have inadequate safety data for pregnancy.

What’s Changing in 2025

There’s hope on the horizon. Pharmacogenomics - testing how your genes affect how you process drugs - is starting to be used in pregnancy care. Imagine knowing, before you even take a pill, whether your body is likely to pass a dangerous amount to your baby. Experts predict this will improve personalized risk prediction by 30-40% in the next five years.

Also, more doctors are working as teams. Your OB, your pharmacist, your mental health provider - they’re talking to each other. Nurses are being trained to flag potential risks before they become problems. And pregnancy registries, though underused, are growing. Over 50 now track medication exposures in real time.

Bottom Line: Stay Informed, Not Afraid

Pregnancy isn’t a time to avoid all medicine. It’s a time to make smarter choices. The goal isn’t zero risk - it’s managing risk. A fever left untreated is more dangerous than acetaminophen. Uncontrolled seizures are more dangerous than carbamazepine. High blood pressure without treatment is more dangerous than labetalol.

Talk to your provider. Don’t Google. Don’t rely on friends. Use trusted resources. Keep a list of everything you take - even ginger tea or melatonin. And remember: if you took a medication before you knew you were pregnant, don’t panic. Most exposures don’t cause harm. But do tell your doctor so they can monitor for any signs of concern.

Your baby’s health matters. But so does yours. The best medicine isn’t the one with no side effects - it’s the one that keeps you alive, healthy, and calm enough to carry your baby to term.