(and most other visible, but non-diagnostic, deformities) are believed to be caused mainly during the 10th to 20th week of gestation. Refinements in diagnostic criteria since 1975 have yielded three distinctive and diagnostically significant facial features known to result from prenatal alcohol exposure and distinguishes FAS from other disorders with partially overlapping characteristics. The three FAS facial features are: A smooth philtrum: The divot or groove between the nose and upper lip flattens with increased prenatal alcohol exposure. Thin vermilion: The upper lip thins with increased prenatal alcohol exposure. Small palpebral fissures: Eye width decreases with increased prenatal alcohol exposure. Measurement of FAS facial features uses criteria developed by the University of Washington. The lip and philtrum are measured by a trained physician with the Lip-Philtrum Guide, a five-point Likert scale with representative photographs of lip and philtrum combinations ranging from normal (ranked 1) to severe (ranked 5). Palpebral fissure length (PFL) is measured in millimeters with either calipers or a clear ruler and then compared to a PFL growth chart, also developed by the University of Washington. Ranking FAS facial features is complicated because the three separate facial features can be affected independently by prenatal alcohol. A summary of the criteria follows: Severe: All three facial features ranked independently as severe (lip ranked at 4 or 5, philtrum ranked at 4 or 5, and PFL two or more standard deviations below average). Moderate: Two facial features ranked as severe and one feature ranked as moderate (lip or philtrum ranked at 3, or PFL between one and two standard deviations below average). Mild: A mild ranking of FAS facial features covers a broad range of facial feature combinations: Two facial features ranked severe and one ranked within normal limits, One facial feature ranked severe and two ranked moderate, or One facial feature ranked severe, one ranked moderate and one ranked within normal limits. None: All three facial features ranked within normal limits. Central nervous system Central nervous system (CNS) damage is the primary feature of any FASD diagnosis. Prenatal alcohol exposure, which is classified as a teratogen, can damage the brain across a continuum of gross to subtle impairments, depending on the amount, timing, and frequency of the exposure as well as genetic predispositions of the fetus and mother. While functional abnormalities are the behavioral and cognitive expressions of the FASD disability, CNS damage can be assessed in three areas: