Development
The earliest observable symptoms of WS include low birth weight, failure to thrive, trouble breastfeeding, nocturnal irritability, and gastroesophageal reflux. Facial dysmorphies thought to be characteristic of the syndrome are also present early in development, as are heart murmurs. Research on the development of the syndrome suggests that congenital heart disease is typically present at an early age, often at the infants first pediatric appointment. Heart problems in infancy often lead to the initial diagnosis of WS.Developmental delays are present in most cases of WS, and include delay of language abilities and delayed motor-skill development. People with WS develop language abilities quite late relative to other children, with the childs first word often occurring as late as 3 years of age. Language abilities are often observed to be deficient until adolescence, in terms of semantics, morphology, and phonology, though not in vocabulary.Williams syndrome is also marked by a delay in the development of motor skills. Infants with WS develop the ability to lift their heads and sit without support months later than typically developing children. These delays continue into childhood, where patients with WS are delayed in learning to walk. In young children, the observed motor delay is around 5–6 months, though some research suggests that children with WS have a delay in development that becomes more extreme with age. Children with motor delays as a result of WS are particularly behind in the development of coordination, fine motor skills such as writing and drawing, response time, and strength and dexterity of the arms.
The physical signs that often indicate a suspected case of WS include puffiness around the eyes, a long philtrum, and a stellate pattern in the iris. Physiological symptoms that often contribute to a WS diagnosis are cardiovascular problems, particularly aortic or pulmonary stenosis, and feeding disturbance in infants. Developmental delays are often taken as an initial sign of the syndrome, as well.If a physician suspects a case of WS, the diagnosis is confirmed using one of two possible genetic tests: Micro-array analysis or the fluorescent in situ hybridization test, which examines chromosome 7 and probes for the existence of two copies of the elastin gene. Since 98-99% of individuals with WS lack half of the 7q11.23 region of chromosome 7, where the elastin gene is located, the presence of only one copy of the gene is a strong sign of WS. This confirmatory genetic test has been validated in epidemiological studies  and has been demonstrated to be a more effective method of identifying WS than previous methods, which often relied on the presence of cardiovascular problems and facial features (which, while common, are not always present).Reliance on facial features to identify WS may cause a misdiagnosis of the condition. Among the more reliable features suggestive of WS are congenital heart disease, periorbital fullness ("puffy" eyes), and the presence of a long, smooth philtrum. Less reliable signs of the syndrome include anteverted nostrils, a wide mouth, and an elongated neck.