Fetal Alcohol Spectrum Disorders (FASD) describes a continuum of permanent birth defects caused by maternal consumption of alcohol during pregnancy, which includes, but is not limited to fetal alcohol syndrome (FAS). Approximately 1 percent of children are believed to have fetal alcohol spectrum disorder.

Over time, as it became apparent through research and clinical experience that a range of effects (including physical, behavioral, and cognitive) could arise from prenatal alcohol exposure, the term Fetal Alcohol Spectrum Disorders, or FASD, was developed to include Fetal alcohol syndrome (FAS) as well as other conditions resulting from prenatal alcohol exposure. There are a number of other subtypes with evolving nomenclature and definitions based on partial expressions of FAS, including Partial Fetal Alcohol Syndrome (PFAS), Alcohol-Related Neurodevelopmental Disorder (ARND), Alcohol-Related Birth Defects (ARBD), and Fetal Alcohol Effect (FAE).

The term Fetal Alcohol Spectrum Disorders is not in itself a clinical diagnosis but describes the full range of disabilities that may result from prenatal alcohol exposure. Currently, Fetal Alcohol Syndrome (FAS) is the only expression of prenatal alcohol exposure that is defined by the International Statistical Classification of Diseases and Related Health Problems and assigned ICD-9 and ICD-10 diagnoses.

A spectrum as it applies to mental disorder is a range of linked conditions, sometimes also extending to include singular symptoms and traits. The different elements of a spectrum either have a similar appearance or are thought to be caused by the same underlying mechanism. In either case, a spectrum approach is taken because there appears to be "not a unitary disorder but rather a syndrome composed of subgroups". The spectrum may represent a range of severity, comprising relatively "severe" mental disorders through to relatively "mild and nonclinical deficits".

In some cases, a spectrum approach joins together conditions that were previously considered separately. A notable example of this trend is the autism spectrum, where conditions on this spectrum may now all be referred to as autism spectrum disorders. In other cases, what was treated as a single disorder comes to be seen (or seen once again) as comprising a range of types, a notable example being the bipolar spectrum. A spectrum approach may also expand the type or the severity of issues which are included, which may lessen the gap with other diagnoses or with what is considered 'normal'. Proponents of this approach argue that it is in line with evidence of gradations in the type or severity of symptoms in the general population, and helps reduce the stigma associated with a diagnosis. Critics, however, argue that it can take attention and resources away from the most serious conditions associated with the most disability, or on the other hand could unduly medicalize problems which are simply challenges people face in life.

Fetal alcohol syndrome (FAS) is a pattern of mental and physical defects that can develop in a fetus in association with high levels of alcohol consumption during pregnancy. Current research also implicates other lifestyle choices made by the prospective mother. Indications for lower levels of alcohol are inconclusive. The current recommendation of both the Surgeon General of the United States and the British Department of Health is to drink no alcohol at all during pregnancy.

Alcohol crosses the placental barrier and can stunt fetal growth or weight, create distinctive facial stigmata, damage neurons and brain structures, which can result in psychological or behavioral problems, and cause other physical damage. Surveys found that in the United States, 10–15% of pregnant women report having recently drunk alcohol, and up to 30% drink alcohol at some point during pregnancy.

The main effect of FAS is permanent central nervous system damage, especially to the brain. Developing brain cells and structures can be malformed or have development interrupted by prenatal alcohol exposure; this can create an array of primary cognitive and functional disabilities (including poor memory, attention deficits, impulsive behavior, and poor cause-effect reasoning) as well as secondary disabilities (for example, predispositions to mental health problems and drug addiction). Alcohol exposure presents a risk of fetal brain damage at any point during a pregnancy, since brain development is ongoing throughout pregnancy.