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Despite advances in methods of its prenatal diagnosis Down's syndrome is still one of the commonest severe defects found at birth. The risk increases with maternal age and prenatal services are traditionally directed at women over the age of 34, but most affected births are to women below the age of 35.
The National Down Syndrome Cytogenetic Register, established in 1989, provides information on the effectiveness of the prenatal diagnosis of this condition. The register is based on reports from the regional cytogenetic laboratories of England and Wales of all karyotypes that would result in a diagnosis of Down's syndrome. Over 95% of all Down's syndrome births and terminations are thought to have been cytogenetically confirmed, although this figure has not been validated. We examined the register's data for 1989.
DATA COLLECTION AND PROCESSING
With the help of the Association of Clinical Cytogeneticists every NHS and private cytogenetic laboratory in the country was contacted. The collaboration of referring obstetricians and paediatricians was sought through the laboratories and through wide publicity about the study. We had no refusals. Data returns were sometimes delayed or incomplete; this has improved since the year of the study.
The laboratories supply data by using a three part form, which does not include any personal identifying data. The form includes the cytogenetic result, indication for investigation, diagnostic test performed, origin of referral, maternal and paternal ages, name of hospital, date of birth or termination of the affected fetus, gestational age at testing or delivery, and town of residence and postcode. After completion most laboratories send the top copy to the register, attach the next copy to the cytogenetic report with a request to the referring clinician to supply any missing information directly (over 80% do so), and retain the last copy. Other laboratories collect the clinical information themselves and forward the complete data set at agreed intervals.
Each register entry is checked for a possible match with any earlier diagnosis, and clinical forms are linked with cytogenetic return forms. Data are coded and entered into the microcomputer by using the statistical package for the social sciences (SSPS …