Fetal alcohol syndrome (FAS) is caused by alcohol exposure during pregnancy. When a pregnant person drinks, alcohol crosses the placenta and reaches the developing baby, where it disrupts cell growth, brain formation, and organ development. There is no known safe amount of alcohol during pregnancy, and no safe trimester in which to drink.
How Alcohol Reaches the Fetus
Alcohol moves from the mother’s bloodstream across the placenta by simple diffusion. It reaches the fetus quickly and easily. The critical problem is what happens next: the fetus cannot clear alcohol efficiently. While a mother’s liver breaks down alcohol at a predictable rate, the amniotic fluid surrounding the baby acts as a reservoir, holding alcohol long after the mother’s blood levels have dropped. Studies measuring alcohol levels in amniotic fluid show that even though peak concentrations there are lower than in maternal blood, the total exposure over time is nearly the same, because clearance from amniotic fluid is roughly half as fast.
This means the fetus effectively soaks in alcohol for longer than the mother experiences its effects. During the second trimester, the fetal liver begins producing its own alcohol-processing enzymes, but this capacity is limited and varies from one pregnancy to another based on genetics.
What Alcohol Does to Developing Cells
Alcohol doesn’t damage a fetus through a single mechanism. It attacks multiple processes at once, which is part of why its effects are so wide-ranging.
One well-studied pathway involves a protein called L1 that helps developing brain cells stick together and migrate to the right locations. Alcohol binds to a specific pocket on this protein, preventing it from doing its job. When L1 can’t guide cells to adhere properly, neurons end up in the wrong places or fail to form the connections they need. This disruption is especially damaging during periods of rapid brain growth.
Alcohol also generates oxidative stress, essentially flooding developing tissues with reactive molecules that damage DNA and cell membranes. It interferes with how the placenta transports essential nutrients like folate to the baby, compounding the direct toxic effects with nutritional deprivation. And it disrupts the signaling pathways that tell embryonic cells when and where to grow, which is why alcohol exposure can affect the face, heart, kidneys, and skeleton in addition to the brain.
Timing Matters: Windows of Vulnerability
Different organs and structures are vulnerable at different points in pregnancy, which is why the type and severity of damage depends heavily on when alcohol exposure occurs.
Weeks 3 Through 6
The facial features associated with FAS, including a smooth ridge between the nose and upper lip, thin upper lip, and small eye openings, develop when alcohol damages a specific group of cells called cranial neural crest cells. These cells are most vulnerable between the third and sixth weeks after fertilization, a period when many people don’t yet know they’re pregnant. These same cells contribute to the developing brain and heart, so early exposure often causes damage beyond facial appearance.
Week 6 Through the Second Trimester
After the brain begins dividing into distinct regions around weeks six and seven, specific structures become targets. The corpus callosum, the bridge connecting the left and right hemispheres of the brain, is particularly vulnerable during this window. Damage to it can impair communication between brain hemispheres, contributing to problems with coordination, problem-solving, and social behavior later in life.
Third Trimester
The final trimester is a period of explosive brain growth. Neurons are multiplying, migrating to their final positions, and forming the synaptic connections that will underlie learning, memory, and behavioral control. The cerebellum, which coordinates movement and balance, undergoes most of its development during this phase. Alcohol exposure in the third trimester can cause significant neurodevelopmental damage even without the characteristic facial features of FAS, because the face has already formed by this point.
This is one reason the full spectrum of alcohol-related disorders is broader than FAS alone. A baby exposed primarily in the third trimester may have serious cognitive and behavioral problems with no visible physical signs.
Why Some Pregnancies Are Affected More Than Others
Not every pregnancy exposed to alcohol results in the same degree of harm. Several factors influence how severely a fetus is affected.
Amount and pattern of drinking: The severity of damage correlates with peak blood alcohol concentration. Binge drinking, which produces sharp spikes in blood alcohol, tends to cause more harm than the same total amount of alcohol spread over many days. That said, no pattern of drinking has been shown to be risk-free.
Genetics: Both the mother’s and the baby’s genetic makeup play a role. Variations in genes that control alcohol-processing enzymes affect how quickly alcohol is broken down and how long the fetus is exposed to it. Some genetic variants lead to slower metabolism, meaning the fetus sits in higher concentrations of alcohol for longer. Twin studies have shown that fraternal twins exposed to the same amount of alcohol in the womb can be affected to very different degrees, underscoring the role of individual genetic susceptibility.
Maternal nutrition: Alcohol impairs nutrient absorption and disrupts the placenta’s ability to deliver vitamins and minerals to the fetus. Two nutrients have received particular research attention. Folic acid supplementation during pregnancy has shown protective effects against growth restriction and certain cardiac problems in alcohol-exposed pregnancies. Choline supplementation at 2 grams daily during the second half of pregnancy has been linked to reduced brain volume loss and improved recognition memory in babies exposed to alcohol prenatally, along with protection against growth restriction at 6 and 12 months of age. These nutrients don’t prevent FAS, but they may reduce the severity of some effects.
Maternal health: Older maternal age, poor overall nutrition, and higher body fat percentages can all influence how alcohol is metabolized and how much reaches the fetus.
The Spectrum Beyond FAS
FAS sits at the most severe end of a broader category called fetal alcohol spectrum disorders (FASD). The full range of conditions caused by prenatal alcohol exposure may affect between 1% and 5% of school-aged children in the United States and some Western European countries.
FAS itself, defined by the combination of facial features, growth problems, and central nervous system damage, occurs in an estimated 6 to 9 out of every 1,000 children based on studies that physically examined school-aged kids. Medical record reviews produce lower numbers, around 1 per 1,000 live births, likely because many cases go undiagnosed.
Alcohol-related neurodevelopmental disorder (ARND) is diagnosed when a child has intellectual disabilities, behavioral problems, or learning difficulties linked to prenatal alcohol exposure but lacks the facial features of full FAS. Children with ARND often struggle with math, memory, attention, judgment, and impulse control. Because there are no visible physical markers, ARND frequently goes unrecognized, and affected children may be misdiagnosed with ADHD or other behavioral conditions.
The distinction between these diagnoses is based on which symptoms are present, not on how much alcohol was consumed. Some children with heavy prenatal exposure show subtle effects, while others exposed to seemingly moderate amounts develop significant impairments. This unpredictability is precisely why health agencies worldwide maintain that no amount of alcohol is safe during pregnancy, and no type of alcohol, whether wine, beer, or liquor, carries less risk than another.