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

When you’re pregnant, every pill, drop, or supplement feels like a 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 hurt your baby. The truth isn’t as simple as ‘all drugs are dangerous’ or ‘everything’s fine.’ It’s messy, complicated, and deeply personal. And it’s not something you should figure out alone.

What Exactly Is a Teratogen?

A teratogen is any substance-medicine, chemical, infection, or even radiation-that can interfere with how a baby develops in the womb. It doesn’t always cause harm, but when it does, the results can be serious: missing limbs, heart defects, brain damage, or even miscarriage. The most famous example is thalidomide, a drug given in the 1950s for morning sickness. By the time it was pulled from shelves, around 10,000 babies had been born with severe limb deformities. 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 not negligible. And the risk isn’t the same for every drug, every woman, or every stage of pregnancy.

When Does Risk Happen? Timing Matters More Than You Think

The first trimester-weeks 1 to 12-is the most dangerous time for teratogenic exposure. That’s when your baby’s organs are forming. Between days 15 and 60 after conception, even a tiny dose of a harmful drug can cause major structural problems. Think of it like building a house: if you mess with the foundation or framing, the whole structure is at risk.

After week 12, the baby’s major organs are mostly done forming. That doesn’t mean drugs are safe, though. In the second trimester, medications can still affect brain development, growth, or how organs function. In the third trimester, the biggest risks shift to things like withdrawal symptoms (if you’ve been taking opioids or antidepressants), altered heart rhythms, or low birth weight.

Some drugs don’t harm the baby directly. Instead, they mess with your body in ways that indirectly hurt the fetus. For example, a blood pressure medication that drops your pressure too low can reduce blood flow to the placenta. Less blood flow means less oxygen and nutrients for your baby. That’s why just stopping a drug without medical advice can be just as risky as keeping it.

Medications With Known Teratogenic Risks

Not all drugs are created equal. Some have clear, well-documented dangers. Here are a few you need to know about:

• Warfarin (a blood thinner): Can cause fetal warfarin syndrome-nose deformities, bone problems, eye damage, and intellectual disability. Risk is highest in the first trimester.
• Carbamazepine (for epilepsy): Increases neural tube defect risk by about 1%. Also lowers vitamin K, which can lead to dangerous bleeding in newborns.
• Methotrexate (used for autoimmune diseases and cancer): A folate blocker. If taken in early pregnancy, it can cause severe birth defects in 10-20% of cases.
• Factor Xa inhibitors (rivaroxaban, apixaban, edoxaban): These newer blood thinners cross the placenta. There’s no antidote if something goes wrong. Avoid them completely during pregnancy.
• Cannabis (THC): Linked to higher risks of low birth weight, preterm birth, and stillbirth. THC stays in breastmilk for up to six days and may affect your baby’s brain development, leading to attention and learning problems later.

These aren’t random examples. These are drugs that have been studied in thousands of pregnancies and consistently shown to cause harm. If you’re taking any of these and you’re pregnant-or thinking about it-talk to your doctor immediately. Don’t stop cold turkey, but don’t wait either.

The Acetaminophen Debate: Why Experts Can’t Agree

Acetaminophen (also called paracetamol) is the go-to painkiller for pregnant women. It’s in dozens of over-the-counter products. For decades, it was considered safe. But now, things are getting messy.

The CDC says some studies have found a possible link between long-term acetaminophen use during pregnancy and higher risks of autism and ADHD in children. But they’re quick to add: no direct cause has been proven.

Meanwhile, the American College of Obstetricians and Gynecologists (ACOG) came out in September 2025 with a strong statement: acetaminophen is still the safest option for managing pain and fever during pregnancy. Why? Because untreated fever can raise the risk of neural tube defects by 20-30%. Untreated pain can lead to high blood pressure, stress, and poor sleep-all bad for pregnancy.

This isn’t a contradiction. It’s a balancing act. The CDC is warning about potential long-term risks based on observational data. ACOG is saying: the known dangers of not treating symptoms are far worse than the uncertain risks of acetaminophen.

So what should you do? Use the lowest effective dose for the shortest time. Don’t take it daily unless your doctor says so. But don’t avoid it out of fear if you need it. A headache or fever that goes untreated is a bigger threat than a single dose of acetaminophen.

What About the Medications You Took Before You Knew You Were Pregnant?

About half of all pregnancies are unplanned. That means a lot of women take medications-antibiotics, antidepressants, even birth control-before they realize they’re pregnant. And panic sets in.

Here’s the truth: most medications don’t cause harm. About 60-70% of drugs used in pregnancy have no known effect on the fetus. If you took something in the first few weeks-before your missed period-it’s likely too early for it to affect development. At that stage, the embryo is either unaffected, or the exposure causes an all-or-nothing effect: it either miscarries or it doesn’t.

That doesn’t mean you should ignore it. But it does mean you shouldn’t assume the worst. Talk to your provider. Get a proper risk assessment. Don’t Google it. Don’t scroll through Reddit threads. Go to someone who can look at your specific meds, your timing, and your health history.

How to Navigate Medication Use Safely

If you’re planning a pregnancy or already pregnant, here’s what to do:

1. Review every medication-prescription, over-the-counter, supplements, and herbal remedies-with your doctor or pharmacist before you conceive.
2. Don’t stop critical meds without talking to your provider. If you have epilepsy, high blood pressure, or depression, stopping your meds can be more dangerous than keeping them.
3. Use trusted resources like MotherToBaby or LactMed. These are science-backed, free services staffed by specialists who answer questions about pregnancy and medication exposure.
4. Ask for alternatives. Sometimes there’s a safer drug. For example, instead of warfarin, doctors may switch you to heparin during pregnancy.
5. Track your use. Keep a list of everything you take, including dosage and dates. This helps your care team make better decisions.

Pharmacists are your secret weapon. They know the details of drug interactions, pregnancy risks, and alternatives better than most doctors. Don’t be shy about asking them.

The Big Problem: We Don’t Know Enough

Here’s the uncomfortable truth: we don’t have solid data on most medications used in pregnancy. Why? Because we can’t ethically run clinical trials on pregnant women. That means 95% of what we know comes from watching what happens after the fact-case reports, registries, or animal studies.

Only about 20% of medications have enough data to give clear safety guidance. The rest? We’re guessing. That’s why the FDA changed the labeling system in 2015. Instead of vague letters (A, B, C, D, X), labels now include detailed narratives: what the data says, what it doesn’t say, and what the risks really look like.

Still, many women are confused. One Reddit user wrote: “My OB said Zofran was safe. Then I read it might cause birth defects. I cried for days.” That’s the reality. Conflicting messages are everywhere.

That’s why you need to rely on experts-not blogs, not social media, not well-meaning friends. Use MotherToBaby. Talk to your OB. Ask your pharmacist. They’ve seen this before.

What’s Next? Better Data, Better Choices

The FDA’s Sentinel Initiative is now tracking 10 million patient records to find patterns in pregnancy medication use. Researchers are also starting to use pharmacogenomics-testing how your genes affect how you process drugs-to predict individual risks. In five years, we may be able to say: “This drug is safe for you, but not for someone else.”

Right now, the system is far from perfect. But it’s improving. And the most important thing you can do is stay informed, ask questions, and never make a decision alone.

If you’re pregnant or planning to be, your health matters-not just for you, but for your baby. Medications aren’t the enemy. Ignorance is. Stay informed. Ask questions. Work with your care team. You’ve got this.