Mechanism
Despite intense research efforts, the exact mechanism for the development of FAS or FASD is unknown. On the contrary, clinical and animal studies have identified a broad spectrum of pathways through which maternal alcohol can negatively affect the outcome of a pregnancy. Clear conclusions with universal validity are difficult to draw, since different ethnic groups show considerable genetic polymorphism for the hepatic enzymes responsible for ethanol detoxification.Genetic examinations have revealed a continuum of long-lasting molecular effects that are not only timing specific but are also dosage specific; with even moderate amounts being able to cause alterations.A human fetus appears to be at triple risk from maternal alcohol consumption:
The placenta allows free entry of ethanol and toxic metabolites like acetaldehyde into the fetal compartment. The so-called placental barrier is practically absent with respect to ethanol. The developing fetal nervous system appears particularly sensitive to ethanol toxicity. The latter interferes with proliferation, differentiation, neuronal migration, axonic outgrowth, integration, and fine-tuning of the synaptic network. In short, all major processes in the developing central nervous system appear compromised. Fetal tissues are quite different from adult tissues in function and purpose. For example, the main detoxicating organ in adults is the liver, whereas the fetal liver is incapable of detoxifying ethanol, as the ADH and ALDH enzymes have not yet been brought to expression at this early stage.
While the "4-Digit Diagnostic Code" includes these criteria for three of its diagnostic categories, it refers to this condition as static encephalopathy. The behavioral effects of ARND are not necessarily unique to alcohol however, so use of the term must be within the context of confirmed prenatal alcohol exposure. ARND may be gaining acceptance over the terms FAE and ARBD to describe FASD conditions with central nervous system abnormalities or behavioral or cognitive abnormalities or both due to prenatal alcohol exposure without regard to growth deficiency or FAS facial features.The following criteria must be fully met for a diagnosis of ARND or static encephalopathy:
Growth deficiency: Growth or height may range from normal to minimally deficient
FAS facial features: Minimal or no FAS facial features present
Central nervous system damage: Clinically significant structural, neurological, or functional impairment in three or more of the Ten Brain Domains
Prenatal alcohol exposure: Confirmed prenatal alcohol exposure;0

Alcohol-related birth defects
Alcohol-related birth defects (ARBD), formerly known as possible fetal alcohol effect (PFAE), was a term proposed as an alternative to FAE and PFAE. The IOM presents ARBD as a list of congenital anomalies that are linked to maternal alcohol use but have no key features of FASD. PFAE and ARBD have fallen out of favor because these anomalies are not necessarily specific to maternal alcohol consumption and are not criteria for diagnosis of FASD. The Canadian guidelines recommend that ARBD should not be used as an umbrella term or diagnostic category for FASD.