Fetal alcohol syndrome (FAS) is diagnosed by evaluating three core areas: distinctive facial features, growth deficiency, and central nervous system abnormalities. There is no single blood test or genetic marker for FAS. Instead, diagnosis relies on a clinical evaluation that combines physical measurements, developmental testing, and, when possible, confirmation that the mother consumed alcohol during pregnancy.
The Three Diagnostic Criteria
According to the American Academy of Pediatrics, a person receives a FAS diagnosis only when all three of the following are present:
- Growth deficiency: height, weight, or both falling at or below the 10th percentile for age, either before or after birth. A head circumference below the 10th percentile also qualifies.
- Three specific facial features: a smooth philtrum (the groove between the nose and upper lip), a thin upper lip, and short palpebral fissures (the horizontal opening of each eye, measured corner to corner).
- Central nervous system problems: structural brain differences, neurological issues, or functional deficits in areas like learning, attention, or executive functioning.
If a child has some but not all of these features, they may receive a diagnosis of partial FAS (pFAS). The AAP notes that diagnostic criteria for conditions along the broader fetal alcohol spectrum, such as alcohol-related neurodevelopmental disorder (ARND), are not yet fully agreed upon by the medical community.
How Facial Features Are Measured
The three facial features associated with FAS are measured with surprising precision. Palpebral fissure length is recorded in millimeters and then converted to a standardized score that reflects how far the measurement falls from the average for a child of that age. A score well below the population norm is one of the diagnostic markers.
Philtrum smoothness and upper lip thinness are each rated on a 5-point scale using a pictorial tool called the Lip-Philtrum Guide. A score of 1 represents a deeply grooved philtrum or a full upper lip (typical), while a score of 4 or 5 represents the smooth philtrum or very thin lip associated with prenatal alcohol exposure. In some clinics, a digital photograph is analyzed by software that traces the outline of the upper lip to produce an objective measurement of thinness.
Growth Deficiency Thresholds
Growth deficiency is assessed by comparing a child’s height and weight to standard growth charts. The key threshold is the 10th percentile: if either height or weight falls at or below this mark at any point across the lifespan, the growth criterion can be met. For more detailed scoring, the University of Washington’s 4-Digit Diagnostic Code breaks growth into four ranks. Both height and weight at or below the 3rd percentile counts as severe. One measurement at or below the 3rd percentile is moderate. Both between the 3rd and 10th percentile is mild. And both above the 10th percentile means the growth criterion is not met.
Central Nervous System Assessment
This is often the most complex part of the evaluation, because brain-related effects of prenatal alcohol exposure can show up in many different ways. The CDC groups central nervous system abnormalities into three categories: structural, neurological, and functional.
Structural Abnormalities
A head circumference below the 10th percentile for age and sex is the most straightforward structural sign. If the child is small overall (both height and weight below the 10th percentile), the head needs to be disproportionately small, at or below the 3rd percentile, to count. Brain imaging can also reveal structural differences such as a reduced or abnormally shaped corpus callosum, cerebellum, or basal ganglia.
Neurological Signs
These include seizures that can’t be explained by another cause, coordination problems, visual-motor difficulties, involuntary eye movements, and trouble with motor control.
Functional Deficits
A child can meet the functional criterion in one of two ways: either through a global cognitive deficit (such as a low IQ score, or significant developmental delay in a child too young for IQ testing) or through deficits in three or more specific areas. Those areas include:
- Cognitive skills: learning disabilities (particularly in math or visual-spatial tasks), uneven skill profiles, or a gap between verbal and nonverbal abilities.
- Executive functioning: difficulty with planning, organization, or strategy. Concrete thinking, poor impulse control, trouble following multi-step directions, and an inability to apply learned information to new situations.
- Motor functioning: delayed milestones, trouble with writing or drawing, clumsiness, balance problems, or poor dexterity. In infants, a weak suck is a common early sign.
- Attention and hyperactivity: being easily distracted, overly active, or unable to complete tasks. Adults often describe these children as “busy” or having trouble with transitions.
The Role of Prenatal Alcohol Exposure
Confirming that alcohol exposure actually occurred strengthens the diagnosis, but it isn’t always possible. Children in foster care or adoptive homes often arrive without reliable pregnancy histories, and the CDC’s guidelines are clear: the lack of confirmed alcohol use should not prevent a FAS diagnosis when all other criteria are present. The only scenario that rules FAS out entirely is documented confirmation that the birth mother consumed no alcohol at all from conception through delivery.
When exposure information is available, it typically comes from the birth mother’s self-reports, accounts from a reliable family member, medical records showing positive blood alcohol levels or alcohol treatment, or legal and social records indicating problem drinking during the pregnancy. A diagnosis made with this documentation is labeled “FAS with confirmed prenatal alcohol exposure.” When exposure can be neither confirmed nor ruled out, the label is “FAS with unknown prenatal alcohol exposure.”
Who Performs the Evaluation
A FAS evaluation usually involves multiple specialists rather than a single appointment. A pediatrician typically coordinates the process, generating referrals and pulling together the findings. A developmental-behavioral pediatrician often handles the assessment of facial features and developmental concerns, particularly in complex cases. These specialists frequently have training in recognizing genetic syndromes that can mimic FAS.
A psychologist or neuropsychologist conducts the cognitive and behavioral testing needed to identify functional deficits. An occupational or physical therapist evaluates fine motor, gross motor, and sensory skills. A social worker may assess the family’s strengths and challenges and help connect them to resources. For young children under age three, early intervention services can provide developmental assessment and monitoring. A geneticist may be brought in when there is a need to rule out genetic conditions with overlapping features.
The 4-Digit Diagnostic Code
One widely used diagnostic framework, developed at the University of Washington, assigns a four-digit code that captures the severity of each diagnostic feature. Each digit reflects one area: growth deficiency, facial features, brain abnormalities, and prenatal alcohol exposure. Each is scored on a scale from 1 (no evidence of the feature) to 4 (strong, classic presentation). So a code of 4-4-4-4 would represent the most severe, clearly confirmed case, while a code of 1-1-1-1 would indicate none of the features are present. This system allows clinicians to capture the full range of fetal alcohol spectrum conditions, not just the most severe end.
Why Diagnosis Is Often Delayed
Many of the behavioral and cognitive signs of FAS overlap with other conditions like ADHD, learning disabilities, or autism spectrum disorder. The characteristic facial features are most distinct in early childhood and can become less obvious with age, making diagnosis harder in adolescents and adults. The stigma around alcohol use during pregnancy can also make exposure history difficult to obtain. These factors combine to create long diagnostic delays for many individuals, which is significant because early identification leads to earlier access to developmental support and educational accommodations.