Nearly every substance a pregnant person takes has some potential to reach the fetus. Estimates suggest that anywhere from 22% to 82% of pregnant women in the U.S. use at least one prescription medication, depending on how use is measured. Some of those medications are necessary and safe, while others carry real risks to fetal development. The same is true for over-the-counter drugs, alcohol, and illicit substances. Understanding how and when these substances affect a pregnancy can help you make informed choices before and during those critical months.
How Substances Reach the Fetus
The placenta is not the airtight barrier many people imagine. Drugs cross it through several routes: passive diffusion (the most common), facilitated diffusion, and active transport by specialized proteins embedded in the placental membrane. The biggest factors that determine whether a drug makes it through are its molecular size, its fat solubility, and how much of it is bound to proteins in the mother’s blood.
Small, fat-soluble molecules slip across most easily. Drugs under about 600 daltons in molecular weight (which includes most common medications) cross readily by diffusion. Larger molecules have a harder time. Protein binding matters too: because the mother’s blood has a higher concentration of the protein albumin than the fetus’s blood, drugs that cling tightly to albumin are less likely to transfer.
The placenta also has built-in defense systems. Efflux transporters on the surface facing the mother’s blood actively pump certain drugs back out, preventing them from reaching fetal circulation. But these defenses aren’t foolproof. Some drugs, particularly weak bases, actually get trapped in fetal blood because fetal circulation is slightly more acidic than the mother’s. Once the drug crosses over and becomes ionized in that acidic environment, it can’t easily diffuse back, so it accumulates.
Why Timing During Pregnancy Matters
A drug’s effect on the fetus depends heavily on when exposure happens. Fetal development follows three broad phases, and each one carries different vulnerabilities.
During the first two weeks after conception (before implantation), exposure tends to follow an all-or-nothing pattern. A toxic exposure either prevents the embryo from implanting at all or causes no lasting damage. The embryonic period, roughly weeks 3 through 8, is when the stakes are highest. This is when the brain, heart, limbs, and other major organs are taking shape. A harmful drug during this window is most likely to cause structural birth defects.
The fetal period runs from week 9 until birth. Organ formation continues throughout this stage, and some systems, particularly the brain and nervous system, remain vulnerable well into the third trimester and even after birth. Drugs taken later in pregnancy may not cause visible structural defects but can still impair growth, brain development, or organ function.
Common Over-the-Counter Medications
Not all OTC drugs are harmless in pregnancy. The most notable example is NSAIDs, the class of pain relievers that includes ibuprofen and naproxen. The FDA warns against using NSAIDs at 20 weeks of pregnancy or later because they can cause a critical blood vessel in the fetus, called the ductus arteriosus, to close prematurely. This vessel is supposed to stay open until birth, allowing blood to bypass the lungs while the baby is still getting oxygen through the placenta. When it closes too early, the result can be serious heart and lung complications. At 30 weeks and beyond, the risk is even higher.
Acetaminophen (Tylenol) has long been considered the safer alternative for pain and fever during pregnancy, though recent research has raised questions about high-dose or prolonged use. For most other OTC products, including decongestants, antihistamines, and herbal supplements, the safety data in pregnancy ranges from limited to nonexistent. The safest approach is to check with a pharmacist or provider before taking anything, even something that seems routine.
Alcohol and Fetal Alcohol Spectrum Disorders
There is no known safe amount of alcohol during pregnancy. Alcohol crosses the placenta freely, and the fetus metabolizes it much more slowly than the mother does, meaning it lingers in fetal blood at higher concentrations for longer.
Prenatal alcohol exposure can lead to a range of conditions grouped under the term fetal alcohol spectrum disorders (FASDs). At the most severe end is fetal alcohol syndrome (FAS), which involves a combination of central nervous system problems, growth deficits, and distinctive facial features: a smooth ridge between the nose and upper lip, a thin upper lip, and small eye openings. Children with FAS often have a smaller head size, problems with attention and hyperactivity, and poor coordination.
Not everyone exposed prenatally to alcohol develops the full syndrome. Partial fetal alcohol syndrome is diagnosed when a child has some but not all of the features. Across the spectrum, the most consistent and damaging effects are neurological: difficulties with learning, memory, impulse control, and social behavior that can persist into adulthood. Because the brain develops throughout the entire pregnancy, no trimester is truly “safe” for drinking.
Opioids and Neonatal Withdrawal
Opioid use during pregnancy, whether from prescription painkillers, heroin, or medication-assisted treatment, can lead to a condition called neonatal opioid withdrawal syndrome (NOWS). After birth, the infant is no longer receiving the opioid through the placenta and goes into withdrawal, typically within the first few days of life.
Symptoms include tremors, excessive crying, poor feeding, diarrhea, and in severe cases, seizures. Affected newborns often need extended hospital stays and may require medication to manage withdrawal safely. The severity depends on the type of opioid, the dose, and how close to delivery the last exposure occurred.
For pregnant people who are already dependent on opioids, abruptly stopping is not recommended because withdrawal itself can be dangerous to the pregnancy. Instead, treatment programs typically transition patients to supervised medication that stabilizes both the mother and the pregnancy, while reducing the risks of illicit drug use.
Cannabis and Brain Development
Cannabis use during pregnancy is increasingly common, partly because of the perception that it’s natural and therefore safe. The evidence says otherwise. The active compounds in cannabis interact with a signaling system in the brain that plays a key role in fetal neural development.
Large epidemiological studies have linked prenatal cannabis exposure to cognitive problems that show up in childhood, adolescence, and even young adulthood. These include impairments in learning, memory, attention, and executive function (the ability to plan, organize, and control impulses). There’s also growing evidence of increased anxiety, emotional dysregulation, and social difficulties in exposed children. Some research suggests these effects may be more pronounced in males, though studies in females remain limited.
Prescription Medications and Balancing Risks
For many people, pregnancy raises a difficult question: is it safer to keep taking a needed medication or to stop it? This is especially relevant for conditions like depression, epilepsy, high blood pressure, and thyroid disorders, where going untreated poses its own risks to both the mother and the baby.
Antidepressants illustrate the tradeoff well. Several SSRIs, including sertraline and fluoxetine, are generally considered reasonable options during pregnancy when depression is moderate to severe. The approach typically involves using a single medication at the lowest effective dose, particularly during the first trimester. One SSRI, paroxetine, is usually avoided because it may slightly raise the risk of heart defects when used early in pregnancy. But untreated depression during pregnancy carries real consequences too: poor nutrition, disrupted prenatal care, preterm birth, and postpartum complications.
Some medications need dose adjustments rather than discontinuation. Thyroid hormone replacement, for instance, often needs to be increased during pregnancy because the body’s demand rises. Other medications are clearly unsafe. The old FDA letter categories (A, B, C, D, X) that once graded drug safety in pregnancy have been replaced by the Pregnancy and Lactation Labeling Rule, which provides a more detailed narrative for each drug. Labels now include a risk summary, clinical considerations, and data sections, along with information about effects on fertility and breastfeeding.
Planning Ahead Before Pregnancy
The most effective time to address medication risks is before conception. A thorough medication review can identify drugs that are known to cause birth defects and find safer alternatives. This is especially important because many of the highest-risk exposures happen in the earliest weeks of pregnancy, often before a person even knows they’re pregnant.
A preconception review typically covers three things: discontinuing or replacing medications with known teratogenic effects, adjusting doses of medications that remain necessary, and confirming that chronic conditions are well controlled on a pregnancy-compatible regimen. If you’re taking any regular medication and considering pregnancy, having this conversation early gives you the best range of options.