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 that the very thing helping you could hurt your baby. This fear isn’t irrational. 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 drugs-some infections, radiation, and chemicals count too. But medications are the most common, and most controllable, source. 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 shelves in 1961, it had caused severe limb deformities in around 10,000 babies worldwide. That tragedy changed medicine forever.

Today, we know teratogens don’t hit every pregnancy the same way. Timing matters more than almost anything else. The first trimester-especially weeks 3 to 8 after conception-is when organs are forming. That’s the window of highest risk for major structural defects like heart problems, cleft palate, or missing limbs. After that, the risks shift. In the second trimester, medications are more likely to affect how organs function, not how they look. By the third trimester, the big concern isn’t birth defects anymore-it’s things like withdrawal symptoms, low birth weight, or changes in brain development.

Medications with Proven Risks

Not all drugs are equal when it comes to pregnancy. Some are outright dangerous. Others are risky only under certain conditions. Here are a few with clear evidence:

• Warfarin: This blood thinner can cause fetal warfarin syndrome if taken early in pregnancy. Babies exposed may have underdeveloped noses, bone abnormalities, eye problems, and intellectual disabilities. Even if you’re on warfarin for a clotting disorder, switching to heparin before conception is often recommended.
• Methotrexate: Used for autoimmune diseases and cancer, this drug blocks folate-a vitamin critical for neural tube development. Taking it in the first trimester raises the risk of neural tube defects by 10-20%. Stopping it at least three months before trying to conceive is standard advice.
• Carbamazepine: A common seizure medication, it carries a 1% risk of spina bifida and can cause bleeding in newborns due to low vitamin K. Folic acid supplements before and during pregnancy help reduce this risk, but the drug itself still requires close monitoring.
• Factor Xa inhibitors (rivaroxaban, apixaban, edoxaban): These newer blood thinners cross the placenta. There’s no antidote if bleeding happens, and human safety data is extremely limited. They’re generally avoided in pregnancy.
• Cannabis (THC): Even if you think marijuana is “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 can also stay in breastmilk for up to six days after use.

The Acetaminophen Controversy

Acetaminophen (Tylenol, paracetamol) is the go-to pain reliever for pregnant women. It’s been considered safe for decades. But now, some studies suggest a possible link between long-term use and increased risk of ADHD or autism in children. The CDC says these studies show an “association,” not proof of cause. Meanwhile, the American College of Obstetricians and Gynecologists (ACOG) came out strongly in September 2025, saying: “The conditions you’re treating-pain and fever-are far more dangerous than any theoretical risk from acetaminophen.”

Untreated fever during pregnancy increases the risk of neural tube defects by 20-30%. Chronic pain can raise stress hormones, which affect fetal development too. So while it’s smart to use the lowest effective dose for the shortest time, ACOG’s message is clear: don’t avoid acetaminophen out of fear. Not treating pain or fever is riskier.

How Medications Actually Harm the Fetus

It’s not just about the drug itself. How it affects your body matters too. Here are the five main ways medications can impact your baby:

1. No effect: About 60-70% of medications don’t cross the placenta in harmful amounts or don’t interfere with development. Many common drugs fall here.
2. Direct damage: The drug reaches the fetus and disrupts cell growth or organ formation-like thalidomide or methotrexate.
3. Indirect harm: 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 its ability to support the baby.
5. Uterine contractions: Certain medications can trigger early labor or preterm birth.

This is why simply saying “this drug is safe” isn’t enough. It depends on your health, your dose, your stage of pregnancy, and how your body processes the drug.

The FDA’s Labeling Change (And Why It Confuses People)

Before 2015, prescription labels used simple letters: A, B, C, D, X. A was “safe,” X was “dangerous.” But that system was misleading. A “C” drug didn’t mean “maybe dangerous”-it just meant animal studies showed risk, but human data was lacking. That’s true for most drugs.

The FDA replaced it with the Pregnancy and Lactation Labeling Rule (PLLR). Now, labels include detailed sections: risks, clinical considerations, and data sources. The problem? Many doctors still think in terms of the old letters. And patients? They’re left confused.

One Reddit user wrote: “My OB said Zofran was fine for nausea. Then I read it might cause birth defects. I didn’t know who to believe.” That’s the reality. Online searches give conflicting answers. Even some medical websites still use the outdated letter system. The truth? Most drugs don’t have enough human data to be labeled “safe.” But that doesn’t mean they’re dangerous.

What You Should Do Before and During Pregnancy

Here’s the practical roadmap:

• Before conception: Review every medication you take-prescription, over-the-counter, supplements, herbal remedies-with your doctor or pharmacist. This includes birth control, antidepressants, and acne treatments like Accutane.
• If you’re already pregnant: Don’t stop any medication without talking to your provider. Stopping epilepsy meds, blood pressure drugs, or antidepressants can be more dangerous than keeping them.
• Use trusted resources: MotherToBaby (run by teratology experts) and LactMed (from the National Library of Medicine) are free, science-backed sources. Avoid WebMD or random blogs.
• Track everything: Write down what you take, when, and why. Even a single dose of ibuprofen or a cold medicine matters.
• Ask about alternatives: Can you use a heating pad instead of painkillers? Can you try acupuncture for nausea? Non-drug options are always the first choice.

And yes-about half of all pregnancies are unplanned. If you took a medication before you knew you were pregnant, don’t panic. Most exposures don’t cause harm. Call your provider or MotherToBaby. They can help you assess the risk based on timing, dose, and drug type.

Why So Little Is Known

You might wonder: why don’t we know more? The answer is simple: we can’t test drugs on pregnant women in controlled trials. Ethical rules prevent it. So 95% of what we know comes from watching what happens after the fact-case reports, registries, and observational studies. That’s messy data. It’s why some drugs are labeled “unknown risk” even after decades of use.

That’s changing slowly. The FDA’s Sentinel Initiative is now tracking 10 million patient records to find patterns. And pregnancy registries-where women voluntarily report their medication use-are growing. But only 15-20% of eligible women join them. More participation means better data.

What’s Next?

The future of pregnancy medication safety is personal. Researchers are starting to use pharmacogenomics-testing how your genes affect how you process drugs-to predict individual risk. Dr. Christina Chambers of MotherToBaby says we could see a 30-40% improvement in risk prediction within five years. That means instead of saying “this drug is risky for everyone,” we’ll say “this drug is risky for people with your genetic profile.”

For now, the best thing you can do is stay informed, ask questions, and never assume a drug is safe just because it’s available without a prescription. And remember: untreated illness in pregnancy can be more dangerous than the medication itself.