Dozens of widely prescribed medications can cause birth defects, organ damage, or fetal death when taken during pregnancy, and the list extends well beyond the handful of drugs most people have heard about. Some, like the acne drug isotretinoin, are so dangerous that pregnancy prevention programs are built around their use. Others, like common blood pressure pills or over-the-counter pain relievers, carry risks that depend heavily on when during pregnancy they are taken. Understanding which drugs are contraindicated, why they cause harm, and how the risk changes with timing and dose is essential for anyone who is pregnant or planning to become pregnant.
How Drugs Reach a Developing Fetus
The placenta is often described as a “barrier,” but it is more accurately a selective filter. Specialized transport proteins on the placental surface actively shuttle some substances toward the fetus while blocking others. Small, fat-soluble molecules tend to cross easily through passive diffusion, while larger or water-soluble drugs may be moved by active transport systems. The net result is that many medications a pregnant person takes will reach the fetal bloodstream at meaningful concentrations.
The placenta’s transport proteins, including a family of efflux pumps and uptake carriers, play a major role in determining how much of a drug actually reaches the fetus.1PubMed Central. An update on placental drug transport and its relevance to fetal drug exposure But here is a wrinkle that complicates the picture: the critical window for many structural birth defects falls during the third through sixth week of embryonic development, before the mature placenta is even fully established. The hemochorial placenta does not reach its mature form until roughly weeks eight through twelve.2PubMed Central. A Barrier to Understanding Teratogenicity: The Critical Periods of Sensitivity for Most Structural Birth Defects Precede the Established Hemochorial Placenta That means the earliest and most vulnerable period of organ formation may involve drug transfer mechanisms that researchers still do not fully understand. It also means that by the time many people realize they are pregnant, the highest-risk window for certain defects has already passed or is underway.
Why the Old FDA Letter Grades Disappeared
For decades, the U.S. Food and Drug Administration classified drugs into pregnancy risk categories labeled A, B, C, D, and X. Category A meant controlled studies showed no risk; Category X meant the drug was absolutely contraindicated. The system was simple, which was its appeal, but also its fatal flaw. A single letter could not communicate that a drug might be safe in the first trimester but dangerous in the third, or that risk depended on dose, or that the evidence was based on animal data with no human studies at all.
In 2015, the FDA replaced the letter system with the Pregnancy and Lactation Labeling Rule, which requires drug labels to include narrative descriptions of risk information, clinical considerations, and supporting data.3PubMed. The 2015 US Food and Drug Administration Pregnancy and Lactation Labeling Rule The new format is more informative but also harder to scan quickly, which is why you still see the old letter categories referenced in many online drug databases and even by some clinicians. If you come across a “Category X” label, the underlying finding is still valid, but the fuller picture now lives in the narrative sections of the drug’s prescribing information.
Blood Pressure Medications That Harm the Fetal Kidneys
ACE inhibitors and angiotensin receptor blockers (ARBs), two of the most commonly prescribed classes of blood pressure medication, are among the clearest examples of pregnancy contraindication. These drugs interfere with the renin-angiotensin system, which in the fetus plays a critical role in kidney development. Exposure during the second and third trimesters can cause fetal kidney failure, dangerously low amniotic fluid, underdeveloped lungs, and skull defects.4PubMed. Fetopathy associated with exposure to angiotensin converting enzyme inhibitors and angiotensin receptor antagonists In severe cases, the damage is fatal.5PubMed. Reversible oligohydramnios in a pregnancy with angiotensin-converting enzyme inhibitor exposure
Both ACE inhibitors and ARBs produce a similar pattern of harm, with the effects most pronounced during the latter half of pregnancy.6PubMed Central. Reversible Fetal Renal Impairment following Angiotensin Receptor Blocking Treatment during Third Trimester of Pregnancy This is why guidelines recommend switching to a pregnancy-compatible blood pressure medication, such as labetalol, nifedipine, or methyldopa, ideally before conception. If you are on an ACE inhibitor or ARB and discover you are pregnant, the standard advice is to stop and switch immediately under medical guidance rather than waiting for your next appointment.
Warfarin and the Blood Thinner Dilemma
Warfarin, the most widely used oral anticoagulant for decades, crosses the placenta readily and is a known teratogen. Exposure during the first trimester can cause a characteristic pattern of birth defects called warfarin embryopathy, which includes nasal cartilage underdevelopment, skeletal abnormalities, and central nervous system defects. Later exposure raises the risk of fetal bleeding and death.7PubMed Central. Management of Anticoagulation in Pregnant Women With Mechanical Heart Valves
The difficulty is that some pregnant patients genuinely need anticoagulation, particularly those with mechanical heart valves where stopping blood thinners entirely can be life-threatening for the mother. Low-molecular-weight heparin does not cross the placenta and is safe for the fetus, but it does not protect the mother as reliably against valve clotting. In one retrospective study, fetal loss was high in the warfarin group (around three quarters of pregnancies), while all women in the low-molecular-weight heparin group experienced serious maternal complications, including valve thrombosis.8PubMed. Low-molecular-weight heparin or warfarin for anticoagulation in pregnant women with mechanical heart valves: what are the risks? A retrospective observational study This is one of the hardest clinical trade-offs in obstetric medicine, and there is no universally right answer. Management typically involves switching between regimens at different stages of pregnancy, with close monitoring throughout.
Anti-Seizure and Psychiatric Medications
Valproic acid, used to treat epilepsy and bipolar disorder, is one of the most well-documented teratogens still in regular clinical use. Roughly one in ten pregnancies exposed to valproic acid results in a major birth defect, including neural tube defects, heart malformations, and cleft palate.9PubMed Central. Valproic Acid in Pregnancy Revisited: Neurobehavioral, Biochemical and Molecular Changes Affecting the Embryo and Fetus in Humans and in Animals: A Narrative Review The risk is dose-dependent, meaning higher doses carry greater danger.10PubMed Central. Major malformations with valproic acid Beyond structural defects, valproic acid also increases the chance of neurodevelopmental problems, including reduced cognitive ability, language delays, and higher rates of autism and ADHD.11PubMed Central. Valproic Acid in Pregnancy Revisited: Neurobehavioral, Biochemical and Molecular Changes Affecting the Embryo and Fetus in Humans and in Animals: A Narrative Review
Lithium, the mainstay treatment for bipolar disorder, poses a different and more nuanced risk. For decades, lithium was considered extremely dangerous in pregnancy based on early registry data that appeared to show a dramatic increase in Ebstein’s anomaly, a rare malformation of the heart’s tricuspid valve.12PubMed Central. Lithium-induced Ebstein’s Anomaly More recent and rigorous research has substantially revised that estimate downward. A large population-based study found that cardiac malformations occurred in about 2.4% of lithium-exposed infants compared with about 1.2% of unexposed infants, giving a roughly 65% relative increase, which is real but much smaller than old estimates suggested. The risk was clearly dose-dependent: daily doses over 900 mg tripled the risk of cardiac malformations, while doses of 600 mg or less showed no statistically significant increase.13PubMed Central. Lithium Use in Pregnancy and the Risk of Cardiac Malformations For some patients with severe bipolar disorder, the risk of untreated illness during pregnancy may outweigh the modest increase in cardiac defect risk at lower lithium doses. This is a conversation that requires careful weighing of individual circumstances.
The broader challenge with epilepsy and psychiatric medications is that the underlying conditions themselves carry pregnancy risks. Uncontrolled seizures can harm both mother and fetus, and untreated severe mental illness increases the risk of poor prenatal care, substance use, and self-harm. Pre-pregnancy counseling is particularly important for these patients, allowing time to switch to lower-risk alternatives when possible.14PubMed. Prescription of hazardous drugs during pregnancy
Isotretinoin and Methotrexate
Isotretinoin, the powerful acne medication sold under brand names like Accutane, is arguably the most tightly controlled teratogen on the market. It causes severe birth defects affecting the face, ears, heart, thymus, and brain, and the mechanism involves disrupting the development of a population of embryonic cells called the neural crest that contributes to all of those structures.15PubMed. Effects of isotretinoin on the behavior of neural crest cells in vitro Because the drug is so dangerous during pregnancy, prescribing it in the United States requires enrollment in a risk management program (iPLEDGE) that mandates pregnancy testing and two forms of contraception.
Methotrexate, widely used for rheumatoid arthritis, psoriasis, ectopic pregnancy, and certain cancers, works by blocking folic acid metabolism, and folic acid is essential for normal fetal development. Exposure during early pregnancy can cause a recognizable pattern of defects involving the skull, limbs, and face, known as fetal methotrexate syndrome.16Reproductive Toxicology. Fetal methotrexate syndrome: A systematic review of case reports It is also associated with heart defects.17PubMed Central. Maternal exposure to methotrexate and birth defects: a population-based study Current guidelines recommend stopping methotrexate at least one to three months before attempting conception, because the drug can persist in tissues for weeks after the last dose.
NSAIDs and the Third-Trimester Cutoff
Nonsteroidal anti-inflammatory drugs like ibuprofen, naproxen, and diclofenac are generally considered low-risk in early pregnancy, but the picture changes sharply after about twenty weeks of gestation. After that point, NSAIDs can cause the ductus arteriosus, a blood vessel in the fetal heart that is supposed to stay open until birth, to close prematurely. A meta-analysis found the risk of ductal closure was roughly fifteen times higher in pregnancies exposed to indomethacin (a potent NSAID) compared to controls.18PubMed. Nonsteroidal antiinflammatory drugs during third trimester and the risk of premature closure of the ductus arteriosus: a meta-analysis This risk applies to the NSAID class broadly, not just indomethacin.19Current Drug Therapy. Oligohydramnios and Intrauterine Premature Closure of Ductus Arteriosus Associated with the Use of NSAIDs During Pregnancy – An Overview of Clinical Evidence
When the ductus arteriosus closes too early, blood flow through the fetal heart is disrupted, potentially leading to heart failure, fluid accumulation, and respiratory problems at birth. A case involving just six doses of diclofenac at 36 weeks resulted in a constricted ductus arteriosus, a dilated right ventricle, and fetal fluid retention.20PubMed Central. Prenatal constriction of the ductus arteriosus following maternal diclofenac medication in the third trimester NSAIDs can also reduce amniotic fluid levels later in pregnancy. Acetaminophen (paracetamol) is generally the preferred pain reliever throughout pregnancy, though even its safety profile is the subject of ongoing research.
Antibiotics and Antifungals
Most commonly used antibiotics, including penicillins and cephalosporins, are considered safe during pregnancy, but a few notable exceptions exist. Tetracyclines, including doxycycline and minocycline, bind to calcium in developing teeth and bones. Exposure during the second or third trimester, when fetal teeth are calcifying, leads to permanent tooth discoloration and can affect bone growth.21PubMed. Tetracycline and other tetracycline-derivative staining of the teeth and oral cavity This is why tetracyclines are avoided after the first trimester.
Among antifungal drugs, fluconazole deserves specific mention. Short courses at low doses (the typical single-dose treatment for a vaginal yeast infection) are generally not considered high risk, but prolonged high-dose fluconazole use during the first trimester has been linked to a distinctive pattern of birth defects involving the face, ears, and limbs.22American Journal of Medical Genetics. Multiple malformation syndrome following fluconazole use in pregnancy: Report of an additional patient The distinction between short-course low-dose and long-term high-dose is clinically important, because many pregnant patients do need treatment for fungal infections. The concern is largely with doses used for systemic fungal disease, not the single 150 mg tablet prescribed for yeast infections.
Thyroid Medication and the Trimester Switch
Hyperthyroidism during pregnancy requires treatment because uncontrolled thyroid hormone levels increase the risk of miscarriage, preeclampsia, and preterm birth. But the two main drugs used to treat it, methimazole and propylthiouracil, each carry their own fetal risks. Methimazole exposure in the first trimester has been associated with a specific set of birth defects, including a skin condition called aplasia cutis and a pattern of malformations sometimes referred to as methimazole embryopathy. Propylthiouracil does not carry the same teratogenic risk in early pregnancy but has been linked to rare but serious maternal liver toxicity with longer use.
A meta-analysis confirmed that propylthiouracil is the safer option during the first trimester.23PLoS ONE. Comparison of the safety between propylthiouracil and methimazole with hyperthyroidism in pregnancy: A systematic review and meta-analysis Current practice typically involves starting with propylthiouracil in the first trimester and then switching to methimazole for the remainder of the pregnancy, though whether the switch itself is better than continuing propylthiouracil alone is not entirely settled.
Herbal Products and the “Natural” Assumption
Many people assume that herbal supplements and “natural” remedies are inherently safer than pharmaceutical drugs, but this assumption does not hold up. Herbal products contain pharmacologically active compounds, and the lack of standardized dosing, regulation, and safety testing makes their risks during pregnancy harder to quantify, not lower. A growing number of women use herbal medicines during pregnancy, often without telling their healthcare provider, which introduces risk that is essentially invisible to the clinical team managing their care.24PubMed. Pregnancy and herbal medicines: An unnecessary risk for women’s health-A narrative review
The evidence base for most herbal products in pregnancy is thin and contradictory. For the majority of commonly used herbs, studies are heterogeneous and give no clear conclusions about safety.25PubMed Central. Herbal Medicines-Are They Effective and Safe during Pregnancy? Some herbs have known uterine-stimulating properties, others may interact with prescription medications, and still others contain compounds that have shown toxicity in animal studies but have never been tested in pregnant humans. The safest approach is to treat herbal products with the same caution as any medication and to disclose their use to your prenatal care provider.
Why the Same Drug Affects Some Pregnancies Differently
One of the more puzzling aspects of drug teratogenicity is that not every exposed pregnancy is affected. Two people can take the same medication at the same dose during the same gestational window, and one baby is born with defects while the other is perfectly healthy. Part of this comes down to genetics. Both the mother’s and the fetus’s genomes influence how a drug is absorbed, metabolized, and transported across the placenta.26PubMed. Genetic susceptibility to teratogens: state of the art
Researchers have identified specific genetic variants that appear to modify teratogenic risk. The most convincing findings involve variants in genes related to serotonin transport, folate metabolism, and glucocorticoid receptors, which were associated with differing susceptibility to birth defects from antidepressants, antiepileptics, and steroid medications respectively.27PubMed Central. Genetic Susceptibility to Drug Teratogenicity: A Systematic Literature Review Pharmacogenetic research has also found that genetic variation in folate metabolism and in placental drug transporters directly relates to birth defect risk from certain drug exposures.28PubMed. Pharmacogenetics of drug-induced birth defects: what is known so far? This field is still young, but it helps explain why blanket statements about drug safety in pregnancy can be misleading. A drug that poses high average risk across all pregnancies may be more dangerous for some individuals than for others based on their genetic makeup.
Do Paternal Medications Matter?
Almost all teratology research focuses on maternal exposures, for the obvious reason that the mother’s bloodstream is the direct route to the fetus. But an emerging area of research asks whether drugs taken by the father before conception might also contribute to adverse outcomes. The theoretical mechanism involves epigenetic changes, modifications to how genes are read without altering the DNA sequence itself, that could be imprinted on sperm during its formation.29PubMed Central. Teratogenesis and the epigenetic programming of congenital defects: Why paternal exposures matter
The practical evidence so far is mostly reassuring. A large Norwegian study of about 340,000 pregnancies found that overall, paternal drug use before conception was not associated with increased risk of birth defects. One exception was diazepam (Valium), where paternal use was linked to a roughly doubled risk of perinatal death and a modest increase in fetal growth restriction, though these findings were from a single study and need replication.30PubMed Central. Effects of preconceptional paternal drug exposure on birth outcomes: cohort study of 340 000 pregnancies using Norwegian population-based databases For now, paternal exposures remain a research interest rather than a well-established clinical concern, but the concept is worth watching as more data accumulates.
How Safety Data Emerges After a Drug Reaches the Market
Pregnant people are almost never included in clinical trials for ethical reasons, which means that when a new drug is approved, its safety during pregnancy is largely unknown. The thalidomide disaster of the late 1950s and early 1960s, which caused severe limb defects in thousands of children, was the event that forced regulatory agencies worldwide to develop systematic testing protocols for reproductive toxicity.31PubMed. Thalidomide: the tragedy of birth defects and the effective treatment of disease But even modern animal testing has significant limitations, since a drug that is safe in rats or rabbits may not be safe in humans, and vice versa.
Because pre-market data are so limited, much of what we know about drug safety in pregnancy comes from post-marketing surveillance. Pregnancy exposure registries, which systematically collect data on pregnancies exposed to specific drugs, have become increasingly important for detecting teratogenic signals and for establishing margins of safety.32PubMed. Pregnancy exposure registries The FDA now frequently requires these registries as a condition of drug approval. Among the types of post-marketing pregnancy safety studies the FDA has required, pregnancy registries are the most common, followed by descriptive safety studies and retrospective cohort analyses.33PubMed. A review of pregnancy and lactation postmarketing studies required by the FDA
This system means that safety information improves over time, but it also means that newly approved drugs carry more uncertainty than older ones. A drug that has been on the market for thirty years with an active pregnancy registry has a much richer safety profile than one approved last year. When a newer medication and an older one are equally effective for a given condition, clinicians managing a pregnant patient will often prefer the one with the longer track record, even if neither has shown a signal of harm.