Introduction: Trisomy 21 is the most common chromosomal abnormality, therefore, screening and diagnosis of this disorder is in the centre of attention worldwide. An efficient screening method is the combined test based on maternal age, ultrasound signs, biochemical markers, and a risk ratio can be calculated based on these data. Aim: The aim of the authors was to determine the causes of missed prenatal diagnosis of Down's syndrome at the 2nd Department of Obstetrics and Gynecology, Semmelweis University. Method: A retrospective study was carried out by collecting data from medical records of mothers who had delivered a newborn with Down's syndrome in the Department between 2008 and 2012. Each medical record was analyzed individually. Results: In most cases the missed diagnosis of Down's syndrome occurred when the expectant mother failed to attend the first trimester screening or did not take the risk of invasive diagnostic procedures needed for fetal kariotyping. Conclusions: Analysis of fetal DNA circulating in maternal plasma can be a solution for those who refuse invasive fetal diagnostics. This test has high sensitivity and very low false positive rate. It has become available since the end of 2011 in the United States and, since the autumn of 2012, in Hungary, too. The test, however, is not reimbursed by national health insurance.
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