Fetal alcohol syndrome (FAS) is a condition caused by alcohol exposure in the womb, and it’s the most severe form of a broader group of conditions called fetal alcohol spectrum disorders (FASDs). Babies born with FAS have a distinct combination of facial features, growth problems, and damage to the central nervous system that affects learning, behavior, and development. Estimates vary depending on how cases are identified, but roughly 1 to 9 out of every 1,000 children in the United States are affected by FAS specifically, while the full range of alcohol-related disorders may affect up to 1 in 20 school-aged children.
How Alcohol Damages a Developing Baby
Alcohol crosses the placenta freely, reaching the baby at roughly the same concentration as in the mother’s blood. Once there, it interferes with fetal development in several ways. It disrupts the normal proliferation and migration of brain cells and can trigger cell death in developing tissue. It also reduces blood flow through the umbilical artery, starving the fetus of oxygen and contributing to growth restriction. On top of that, alcohol inhibits protein synthesis, alters hormone levels, and damages the blood vessels forming in the developing brain.
The timing of exposure matters for which problems develop, though no trimester is safe. Alcohol use in the first three months of pregnancy is most strongly linked to the characteristic facial abnormalities of FAS, because that’s when facial structures are forming. But the brain develops throughout pregnancy and remains vulnerable the entire time. Growth restriction and behavioral problems can result from exposure at any point. There is no known safe amount, type, or timing of alcohol use during pregnancy.
The Three Hallmarks of FAS
A diagnosis of FAS requires all three of the following to be present: specific facial features, growth deficiency, and central nervous system abnormalities.
Facial Features
The three facial characteristics are a smooth philtrum (the groove between the nose and upper lip is flat rather than ridged), a thin upper lip, and short palpebral fissures (the eye openings are smaller than expected). All three must be present for a diagnosis of FAS. Clinicians measure these against standardized guides that account for racial and ethnic norms, because normal facial proportions vary across populations. These features can be subtle, which is one reason many cases go undiagnosed.
Growth Deficiency
Babies with FAS are small. The diagnostic threshold is a height or weight at or below the 10th percentile, meaning the child is smaller than 90% of peers of the same age and sex. This can show up at birth as low birthweight or emerge during infancy and childhood. Growth deficiency is tracked across two windows: prenatal (measured at birth) and postnatal (everything after). Children whose height falls at or below the 3rd percentile have the most severe growth restriction.
Central Nervous System Problems
This is the most consequential category and takes several forms. Structurally, babies with FAS often have a smaller head circumference (below the 10th percentile), reflecting reduced brain size. Brain imaging can reveal changes in the corpus callosum, the thick bundle of fibers connecting the two brain hemispheres, as well as in the cerebellum and other deep brain structures. The white matter tracts that carry signals between brain regions also show abnormalities.
Functionally, these structural differences translate into real difficulties. Children with FAS commonly struggle with overall intellectual ability, attention, working memory, impulse control, processing speed, and social skills. Some experience motor delays or seizures. These deficits aren’t always obvious in the first months of life but become increasingly apparent as a child reaches developmental milestones, enters school, and faces greater cognitive demands.
FAS Versus Other Alcohol-Related Conditions
FAS sits at the most severe end of the fetal alcohol spectrum. Not every baby exposed to alcohol in the womb develops full FAS. Other conditions on the spectrum include:
- Partial fetal alcohol syndrome (pFAS): The child has a confirmed history of prenatal alcohol exposure and some, but not all, of the facial features, along with either a growth problem or nervous system abnormalities.
- Alcohol-related neurodevelopmental disorder (ARND): There are no distinctive facial features, but the child has intellectual disabilities or significant problems with behavior, learning, memory, attention, or impulse control tied to prenatal alcohol exposure.
ARND is particularly tricky because without the visible facial features, the connection to alcohol is harder to recognize. Many children with ARND go undiagnosed or are misdiagnosed with other behavioral or learning conditions.
How FAS Is Identified
There’s no single blood test that confirms FAS after birth. Diagnosis relies primarily on physical examination, growth measurements, and neurodevelopmental assessment. Clinicians use standardized lip-philtrum guides to rate the facial features on a scale, measure the eye openings with a ruler in millimeters, and compare all measurements against published norms for the child’s age, sex, and racial background.
Screening for prenatal alcohol exposure has traditionally depended on maternal self-reporting, which is unreliable for obvious reasons. Researchers have explored biological markers as alternatives. One approach analyzes meconium, the dark stool a newborn passes in the first days of life, for fatty acid ethyl esters. These are byproducts of alcohol metabolism that accumulate in meconium over the second and third trimesters. While this testing has shown promise in high-risk populations, it isn’t yet part of routine newborn screening.
What Early Diagnosis Changes
Early diagnosis is one of the strongest protective factors for children with FAS. Children identified early can be enrolled in early intervention services, which are federally mandated programs available from birth through age 3. These services include therapies targeting speech, motor skills, and social interaction. In most states, a diagnosis of FAS makes a child immediately eligible.
Even without a formal FAS diagnosis, infants who show developmental delays or are known to have been exposed to alcohol may qualify for services under the Individuals with Disabilities Education Act, which covers children at risk of developmental delays. Beyond therapy, early diagnosis helps families and educators understand why a child may behave or learn differently, which shapes the kind of support the child receives at home and in school. Children diagnosed early can be placed in appropriate educational settings and connected with social services that make a measurable difference in long-term outcomes.
What Daily Life Looks Like
The effects of FAS are permanent, but their severity varies widely. Some children have mild learning differences that respond well to support, while others face significant intellectual disability and lifelong difficulty with independent living. Common challenges include trouble following multi-step instructions, difficulty understanding cause and effect, poor impulse control, and struggles with social relationships. Math tends to be especially difficult. Many children with FAS are friendly and eager to please but repeatedly misread social cues, which can lead to frustration and isolation as they get older.
Physical growth often remains below average throughout childhood. Some children catch up partially, but many stay small for their age. The facial features that define FAS can become less distinctive over time, particularly in adolescence and adulthood, which occasionally leads to the mistaken impression that a child has “outgrown” the condition. The brain-based effects, however, persist. The goal of treatment is not to cure FAS but to build skills, reduce secondary problems like school failure or social difficulties, and help the child reach their full potential within their own capabilities.