Introduction: Down syndrome screening has become an important part of antenatal care and is performed for pregnant women worldwide. In this study, 45 statistically accurate cases of false negatives in the first trimester screening tests have been examined statistically, so that by paying attention to some criteria, we can find out how many of these cases can be detected and how much each criteria  can affected by this protocol.
Type and location of the study: The present study is a retrospective study on 197210 Iranian pregnant women who in the first trimester of pregnancy for Down Syndrome screening tests referred to a private laboratory referring to prenatal screening tests or partner laboratories of this laboratory had been done. All of these pregnant women belonged to the city of Tehran, surrounding towns and 20 different provinces from all over Iran.
Objectives of the study: The main objectives of the study are to obtain the effect of each indicator in increasing statistical indicators such as: detection rate, positive screen rate, false negative rate and OAPR (Odds of being Affected given a Positive Result).
Methods: First trimester screening test to determine the risk of anoploid disorders for 197210 pregnant women who from April 2015 to March 2017 to a private reference laboratory for prenatal screening tests or laboratories of the above laboratory in 20 provinces from all over Iran. 6 to 8 months later, patients were contacted and statistical data related to each were recorded and analyzed.
Results: The mean age of the study group was about 28.7 years and the ratio of women over 35 years to all women was 16.1%. Based on the findings of the above study, if we set the high risk cut-off for Down syndrome equal to 1: 100 or more, the detection power is 69.4% (false positive 1.7%) and in this cut-off, OAPR (Odds of being Affected) given a Positive Result) will be equal to 1:16 (that is, one in every 16 people referred for diagnostic tests will have Down Syndrome). If we consider this cut-off of 1: 1500, the false positives reach 12.8%, while about 10% is added to the detection power and the OAPR reaches 1:87.
Discussion: Considering a cut-off equal to or greater than 1: 250, 3.3% of patients were in this group and in 259 of 304 cases Down syndrome was diagnosed (DR = 85.2%) and patients whose risk was between 1 : 251 to 1: 1500 (which in the previous national protocol of Iran were considered as an intermediate risk group for aneuploid disorders), 19326 people (equivalent to 9.8% false positive) had to be forced to do more to 24 cases of Down syndrome They are diagnosed with a 7.9% increase in strength, and a total of 87 people should be advised to d