| http://www.w3.org/ns/prov#value | - Estimates of the maternal age-related risk for trisomy 21 at birth are based on two surveys with almost complete ascertainment of the affected patients; in a survey in South Belgium, every neonate was examined for features of trisomy 21 and, in a survey in Sweden, information was verified using several sources such as hospital notes, cytogenetic laboratories, genetic clinics and schools for the me
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