Nearly every drug taken during pregnancy reaches the developing fetus to some degree, primarily through passive diffusion across the placenta. The consequences range from negligible to catastrophic, depending on the substance, the dose, and the stage of pregnancy at which exposure occurs. Some drugs cause recognizable structural birth defects; others leave subtler marks on brain wiring, organ function, or even the chemical tags that regulate gene activity for years after birth. The story is more nuanced than a simple list of “safe” and “unsafe” medications, because the placenta is not just a passive barrier and timing changes everything.
How Drugs Cross the Placenta
The placenta is the only interface between the mother’s bloodstream and the fetus, and it was once imagined as an effective drug filter. In reality, the vast majority of drugs that enter a pregnant person’s circulation will also enter fetal circulation. Passive diffusion is the main route: small, fat-soluble, uncharged molecules slip across the placental membrane with relative ease. Some compounds are actively pumped across by transporter proteins on the placental surface, and a few use facilitated diffusion. Other transfer routes like pinocytosis exist but are too slow to meaningfully change fetal drug levels.1SpringerLink. Drug transfer and metabolism by the human placenta
How much of a drug ultimately reaches the fetus also depends on enzymes inside the placenta itself. Placental tissue contains phase I and phase II drug-metabolizing enzymes, including members of the cytochrome P450 family, that can break down or modify drugs before they pass through. The activity of these enzymes fluctuates throughout pregnancy, so the same drug taken in the first trimester may cross in different amounts than it would in the third.2PubMed. Altered gene expression and activity of human placental drug-metabolizing cytochrome P450 enzymes across gestation
The Placenta’s Built-In Drug Pump
One of the more remarkable protective features of the placenta is a protein called P-glycoprotein (P-gp), which sits on the surface of placental cells facing the fetus and actively pumps certain drugs back into the mother’s bloodstream. P-gp functions as an efflux transporter: it recognizes a wide range of foreign substances and ejects them from the fetal side of the placenta before they can accumulate.3Reproductive Toxicology. P-glycoprotein in the placenta: Expression, localization, regulation and function
The importance of this pump has been demonstrated in animal studies. When researchers gave P-gp substrate drugs to pregnant mice that lacked the gene for P-gp, fetal drug concentrations skyrocketed: depending on the drug, fetuses without functional P-gp absorbed anywhere from about two-and-a-half to sixteen times more drug than fetuses with normal P-gp. The same effect could be triggered by co-administering a drug that blocks P-gp activity, which has real clinical implications: if a pregnant person takes two medications and one of them inhibits P-gp, the second drug could reach the fetus in much higher amounts than expected.4PubMed Central. Absence or pharmacological blocking of placental P-glycoprotein profoundly increases fetal drug exposure
Why Timing Changes Everything
Fetal development follows a tightly choreographed sequence, and a drug’s potential to cause harm depends heavily on when during that sequence the exposure happens. The first two weeks after conception are sometimes called the “all-or-nothing” period: a toxic insult either prevents the embryo from implanting or is repaired without lasting damage. The period of greatest vulnerability to structural birth defects runs roughly from the third through the eighth week of gestation, when the major organ systems are being laid down. An exposure during this window can disrupt the formation of the heart, limbs, face, or neural tube.
After the first trimester, the risk of gross structural malformations drops, but fetal organs are still maturing. The brain, kidneys, and lungs continue developing well into the third trimester and remain sensitive to chemical insults. Research has shown that drug exposure during pregnancy can affect the fetus through multiple pathways, including oxidative stress, disruption of placental function, and altered levels of maternal glucocorticoids reaching the fetus. These effects may also differ by sex and can sometimes persist across multiple generations through changes in how genes are regulated.5PubMed Central. Prenatal medication exposure and epigenetic outcomes: a systematic literature review and recommendations for prenatal pharmacoepigenetic studies
Antiepileptic Drugs and the Dose Problem
Valproic acid, widely used to treat epilepsy and bipolar disorder, is one of the best-studied examples of a drug that causes structural birth defects. Compared with other antiepileptic medications, valproate roughly doubles to triples the risk of major malformations, and compared with the general population, the risk is nearly four times higher. The risk is clearly dose-dependent: it begins to climb at daily doses around 600 mg and becomes especially pronounced above 1,000 mg per day.6PubMed Central. Major malformations with valproic acid
The dose-response pattern seen with valproate illustrates a broader principle: many teratogenic drugs do not follow a simple on/off switch. Instead, risk scales with how much drug the fetus is exposed to over a given time window. This makes clinical decisions genuinely difficult for conditions like epilepsy, where uncontrolled seizures during pregnancy also pose serious risks to both mother and fetus. The goal becomes finding the lowest effective dose or switching to a less teratogenic medication before conception when possible.
Blood Pressure Medications and the Fetal Kidney
ACE inhibitors, a class of drugs commonly prescribed for high blood pressure, present a different pattern of harm. First-trimester exposure appears to carry relatively modest risk, but exposure in the second and third trimesters can cause a recognizable set of problems centered on the fetal kidneys. Because the fetal renin-angiotensin system plays a critical role in kidney development and blood pressure regulation, blocking it with an ACE inhibitor can lead to kidney failure in the newborn, abnormally low amniotic fluid, underdevelopment of the skull bones, lung hypoplasia, and growth restriction. In severe cases, the outcome is fetal or neonatal death.7PubMed. An overview of the influence of ACE inhibitors on fetal-placental circulation and perinatal development
The mechanism involves both a direct effect on the fetus’s own developing blood pressure regulation and an indirect route: maternal blood pressure drops, reducing blood flow to the placenta and cutting off oxygen and nutrient delivery.8PubMed Central. Taking angiotensin-converting enzyme inhibitors during pregnancy: is it safe? This makes ACE inhibitors a case where the drug’s intended effect in the mother, lowering blood pressure, becomes a source of harm to the fetus through reduced placental perfusion.
Antidepressants and Newborn Lung Pressure
SSRIs are among the most commonly prescribed medications during pregnancy, which makes even small increases in risk clinically significant. The main concern that has emerged over the past two decades is a condition called persistent pulmonary hypertension of the newborn (PPHN), where blood pressure in the newborn’s lungs stays dangerously high after birth and the baby struggles to oxygenate its blood. One early study found that SSRI exposure after the twentieth week of pregnancy was associated with about a sixfold increase in the odds of PPHN, while exposure before twenty weeks or use of non-SSRI antidepressants carried no increased risk.9PubMed. Selective serotonin-reuptake inhibitors and risk of persistent pulmonary hypertension of the newborn
Later and larger studies have confirmed the association but placed the risk somewhat lower. A large Nordic cohort study found that late-pregnancy SSRI exposure roughly doubled the odds of PPHN, with an absolute risk of about 3 per 1,000 live births compared with a background rate of roughly 1.2 per 1,000. The increased risk appeared similar across sertraline, citalopram, paroxetine, and fluoxetine.10PubMed. Selective serotonin reuptake inhibitors during pregnancy and risk of persistent pulmonary hypertension in the newborn: population based cohort study from the five Nordic countries A separate study that accounted for maternal depression itself found an approximately fourfold increased odds of PPHN with SSRI use in the second half of pregnancy but no significant association with SNRI use over the same window.11PubMed Central. SSRI and SNRI use during pregnancy and the risk of persistent pulmonary hypertension of the newborn
The discrepancy between studies matters here. The absolute risk remains low, and untreated severe depression during pregnancy carries its own risks, including preterm birth and low birth weight. Decisions about continuing or stopping SSRIs during pregnancy are genuinely individualized, which is part of why this question resists a clean public-health recommendation.
Alcohol and the Developing Brain
Alcohol is arguably the most thoroughly documented teratogen, responsible for a spectrum of developmental effects collectively known as fetal alcohol spectrum disorders. Unlike many prescription drugs where risk is confined to a specific window or organ system, alcohol threatens fetal development across all three trimesters and affects multiple organ systems, with the brain bearing the heaviest burden.
Ethanol triggers massive neuron death in the immature fetal brain.12PubMed Central. Alcohol and the Developing Brain: Why Neurons Die and How Survivors Change One mechanism involves mitochondrial damage: in utero ethanol exposure sets off a cascade of events in fetal brain cells consistent with a type of programmed cell death driven by mitochondrial dysfunction and oxidative stress.13Alcoholism: Clinical and Experimental Research. In Utero Ethanol Exposure Causes Mitochondrial Dysfunction, Which Can Result in Apoptotic Cell Death in Fetal Brain No safe threshold of alcohol consumption during pregnancy has been established, which is why public health guidance in most countries recommends complete abstinence.
Nicotine, Opioids, and Cannabis
Nicotine exposure during pregnancy affects fetal growth through two simultaneous routes. It raises maternal blood pressure and heart rate while constricting the uterine arteries, reducing blood flow to the placenta. At the same time, carbon monoxide from cigarette smoke binds to hemoglobin, reducing the blood’s ability to carry oxygen. The result is a fetus that receives less blood and less oxygen from the blood it does receive, which contributes to the well-documented association between smoking and low birth weight.14PubMed. The maternal and fetal physiologic effects of nicotine
Opioid exposure in utero leads to physical dependence in the fetus. After birth, the newborn loses its supply and enters withdrawal, a condition called neonatal opioid withdrawal syndrome (NOWS). Symptoms include irritability, tremors, poor feeding, rapid heart rate, high blood pressure, fever, and weight loss.15PubMed. Neonatal opioid toxicity: opioid withdrawal (abstinence) syndrome with emphasis on pharmacogenomics and respiratory depression The severity varies widely between infants and appears to depend on factors including the specific opioid, the dose, the duration of exposure, and the newborn’s own capacity to metabolize the drug.
Cannabis presents a more recently studied concern. THC binds to cannabinoid receptors in the fetal endocannabinoid system, which plays a role in guiding normal brain wiring during development. Exposure to THC during this process can alter neurodevelopment and potentially re-route cannabinoid signaling circuits in the developing brain.16PubMed. Prenatal cannabis exposure – The “first hit” to the endocannabinoid system Research suggests that phytocannabinoids can interfere with the precisely timed endocannabinoid signals that help decide whether developing cells become neurons or support cells, potentially setting up the biological basis for psychiatric problems later in life.17PubMed Central. Physiological Rules of Endocannabinoid Action During Fetal and Neonatal Brain Development
Long-Term Neurodevelopmental Effects
Some of the most consequential effects of prenatal drug exposure do not show up at birth. Prenatal cocaine exposure, for instance, rarely causes the dramatic birth defects that were predicted during the “crack baby” panic of the 1980s. What research has actually shown is subtler but still significant: long-lasting effects on attention, emotional regulation, and the brain’s reward circuitry. These effects appear to be mediated through areas of the brain rich in dopamine receptors, particularly the prefrontal cortex. Both animal models and clinical studies report that prenatally exposed children show difficulties with attention tasks and emotional reactivity.18PubMed Central. Prenatal exposure to drugs: effects on brain development and implications for policy and education
This pattern of delayed and diffuse neurobehavioral effects extends beyond cocaine. Researchers have concluded that in utero drug exposure broadly can have lasting implications for brain structure and function, but better work is needed to define sensitive periods, dose-response relationships, and long-term outcomes including learning disabilities, mental health disorders, and reduced neural adaptability.19PubMed Central. Developmental consequences of fetal exposure to drugs: what we know and what we still must learn The honest assessment is that we know certain exposures alter brain development but are still working out the precise nature and severity of the long-term consequences for many substances.
Epigenetic Marks That Outlast the Exposure
Beyond direct structural or functional damage, prenatal drug exposure can change how genes are expressed without altering the DNA sequence itself. The main mechanism studied in humans is DNA methylation, where chemical tags are added to or removed from DNA, dialing genes up or down. Researchers have found that many of the pathways through which in utero substance exposure affects fetal development are epigenetic in nature.20PubMed Central. Prenatal substance exposure and offspring development: Does DNA methylation play a role?
A systematic review of prenatal medication exposure and epigenetic outcomes found that most studies did detect significant differences in DNA methylation after drug exposure, measured in cord blood, placental tissue, or cells from the baby’s cheek. However, there was almost no consistency across studies that looked at the same medications using the same laboratory methods.5PubMed Central. Prenatal medication exposure and epigenetic outcomes: a systematic literature review and recommendations for prenatal pharmacoepigenetic studies In other words, the field is confident that epigenetic changes happen but cannot yet reliably say which specific changes matter for which drugs. The clinical significance of most observed methylation differences remains unclear.
When the Father’s Drug Use Matters Too
Most discussions of prenatal drug exposure focus on what the pregnant person takes. But a growing body of research suggests that a father’s drug use before conception can also leave marks on offspring. The mechanism runs through sperm: drugs can alter the epigenetic landscape of sperm cells, and those alterations can be carried into the embryo at fertilization.
Animal studies have found that offspring sired by drug-exposed males show altered histone modifications and DNA methylation in genes related to brain signaling, including glutamate and neurotrophic factor pathways. In some cases, the epigenetic modifications found in the father’s sperm were maintained in the brains of his offspring.21PubMed Central. Who’s your daddy? Behavioral and epigenetic consequences of paternal drug exposure A study on cannabinoid receptor activation found that paternal exposure to a synthetic cannabinoid caused changes in the sperm epigenome that were then transferred to embryonic tissues, accounting for observed growth defects in the embryos and placentas.22Scientific Reports. Paternal activation of CB2 cannabinoid receptor impairs placental and embryonic growth via an epigenetic mechanism Emerging evidence from human data suggests that both paternal pre-conception and maternal prenatal substance use can alter offspring epigenetic regulation and influence neurodevelopment and mental health outcomes later in life.23PubMed Central. An epigenetic synopsis of parental substance use
Why the Research Is Harder Than It Looks
Studying the effects of prenatal drug exposure is plagued by confounding factors that make clean conclusions difficult. Fetal exposure to one drug rarely occurs in isolation. People who use illicit drugs during pregnancy often use more than one substance, and they frequently face additional risk factors including inadequate prenatal care, poor nutrition, chronic stress, and unstable living conditions. Separating the effect of the drug itself from these overlapping risks requires careful study designs that are expensive and ethically constrained.
A key example is cocaine. Early media reports in the 1980s and 1990s predicted devastating outcomes for children exposed prenatally to crack cocaine. When researchers later conducted more rigorous studies that controlled for polydrug use, medical complications, and social risk factors, the severe consequences attributed specifically to cocaine exposure largely did not hold up.24Clinics in Perinatology. Interpreting Research on Prenatal Substance Exposure in the Context of Multiple Confounding Factors That does not mean cocaine is harmless in pregnancy. It means the early picture was exaggerated by the failure to account for the chaotic circumstances that typically accompany crack use. The lesson applies broadly: headlines about any single drug’s prenatal effects should be read with an awareness that isolating one substance’s contribution is genuinely difficult.
GLP-1 Receptor Agonists and Other Emerging Questions
As new medications become widely used, new pregnancy-exposure questions arise. GLP-1 receptor agonists, originally developed for type 2 diabetes and now massively popular for weight loss, are not approved for use during pregnancy, and manufacturers recommend stopping them before conception. But with millions of reproductive-age people taking these drugs, unintended early-pregnancy exposures are inevitable.
Early data is cautiously reassuring for first-trimester exposure. A multicenter observational study across six European teratology information services found that first-trimester GLP-1 receptor agonist exposure was not associated with an increased risk of major birth defects compared with either a diabetic or an overweight control group.25PubMed Central. Use of GLP1 receptor agonists in early pregnancy and reproductive safety However, the available safety data remains thin, and there is essentially no information on later-pregnancy exposure. Pre-conception counseling about stopping GLP-1 receptor agonists before becoming pregnant is considered critical as use of these drugs continues to expand.26PubMed Central. GLP-1 receptor agonist therapy and pregnancy: Evolving and emerging evidence
How Pregnancy Drug Labels Have Changed
For decades, the US Food and Drug Administration used an A/B/C/D/X letter grading system to indicate a drug’s risk during pregnancy. The system was widely criticized for being oversimplified and misleading: a “Category C” label, for instance, could mean almost anything from “not studied in humans” to “some concern in animal studies.” In 2015, the FDA discontinued the letter categories and replaced them with the Pregnancy and Lactation Labeling Rule, which requires manufacturers to provide narrative descriptions of risk information, clinical considerations, and available data instead of a single letter grade.27PubMed. The 2015 US Food and Drug Administration Pregnancy and Lactation Labeling Rule
The intent was to give healthcare providers nuanced information they could actually use in clinical decision-making. Whether the new system has achieved that goal is another question. Researchers have examined whether newly approved drugs under the revised labeling framework actually include data-driven pregnancy and lactation recommendations, and the findings suggest the quality and completeness of the information remains uneven.28PubMed. Availability of Safe and Effective Therapeutic Options to Pregnant and Lactating Individuals Following the United States Food and Drug Administration Pregnancy and Lactation Labeling Rule Pregnant people are routinely excluded from clinical trials for ethical reasons, which means the pregnancy sections of most drug labels are based on limited human data and animal studies. The result is that many medications carry labels that effectively say “we don’t know” in more or fewer words.
When the Fetus Is the Patient
Not all fetal drug exposure is accidental or unwanted. In some cases, the entire point is to treat the fetus. Fetal arrhythmias can be managed by giving antiarrhythmic drugs to the mother, knowing the medication will cross the placenta and reach the fetal heart. Corticosteroids are routinely given to pregnant people at risk of preterm delivery to accelerate fetal lung maturation, dramatically reducing the risk of respiratory distress syndrome in premature babies. Certain fetal blood disorders and endocrine conditions can also be treated through medications delivered via the mother’s circulation.
This flips the usual risk-benefit calculation. Instead of weighing fetal risk against maternal benefit, clinicians weigh fetal benefit against maternal side effects. Computational pharmacokinetic models are increasingly being used to predict how much of a drug will actually reach the fetus, helping clinicians choose doses that achieve therapeutic levels in the fetal compartment without over-exposing the mother. Recent modeling work on metformin, for example, has incorporated placental transporter activity and passive permeability data to simulate fetal-to-maternal concentration ratios across pregnancy, allowing better prediction of fetal exposure than older, simpler models.29PubMed Central. Comparison of Metformin PBPK Models Incorporating Placental Transfer to Predict Fetal and Maternal Exposure As these tools improve, they may make it possible to fine-tune drug doses with the fetus specifically in mind, rather than relying on rough estimates of what fraction of a maternal dose will cross the placenta.