PURPOSE: To examine the accuracy of the Fetal Medicine Foundation algorithm used to adjust the risk for trisomy 21 after ultrasound findings detected or not at the time of the anomaly scan.METHODS: This was a retrospective study of all amniocenteses performed in a single centre, in singleton pregnancies, between 1998 and 2008. Maternal demographic characteristics, second-trimester ultrasound findings, indications for amniocentesis and karyotype results were reviewed. The algorithm introduced by the Fetal Medicine Foundation was used to calculate the patient specific risk for trisomy 21 using the age related background risk and anomaly scan findings. Expected trisomy 21 cases based on these risks was compared with the actual karyotype results.RESULTS: Overall, 4,511 cases of singleton pregnancies that underwent second-trimester amniocentesis were reviewed. In 572 cases (12.7%), there were markers of chromosomal abnormality and no previous screening for trisomy 21. The expected number of trisomy 21 cases based on maternal age for this population of 572 cases was 1. The expected number of chromosomal abnormalities after adjusting for ultrasound findings based on the algorithm introduced by the Fetal Medicine Foundation was 6.9 (95% confidence interval 3.4-14.3). After karyotyping, in this population there were 6 cases of trisomy 21, 1 case of Trisomy 18 and 1 case of XXY.CONCLUSIONS: The algorithm that adjusts the age related risk of trisomy 21 according to second-trimester anomaly scan findings is very accurate in predicting the modified risk.