To examine the performance of first-trimester combined screening after adding the specific algorithms for trisomies 18 and 13 in the Down syndrome screening program for chromosomal abnormalities other than trisomy 21 and to determine the outcomes of such pregnancies.
A retrospective study.
Oulu University Hospital, Finland.
Pregnant women (n=56 076) participating voluntarily in first-trimester combined Down syndrome screening in Northern and Eastern Finland during the study period 1 June 2002 to 31 December 2008.
The data of all known cases of chromosomal abnormalities other than trisomy 21 were collected.
Risk algorithms for trisomies 21, 18 and 13 were used for the calculation of patient-specific risks for certain chromosomal abnormalities. Algorithms were based on maternal age, crown-rump length, nuchal translucency, and measurement of free ß-human chorionic gonadotrophin and pregnancy-associated plasma protein-A. Detection rates and false-positive rates were calculated.
A total of 27 cases of trisomy 18, 11 cases of trisomy 13 and 30 cases of other chromosomal abnormalities were analyzed. The algorithm for Down syndrome detected 55.6% of trisomy 18 cases, 36.4% of trisomy 13 cases and 60.0% of other chromosomal abnormalities. When specific risk algorithms were added, the detection rates improved for trisomy 18 (74.0%) and for trisomy 13 (54.5%), with only a slight increase of the false-positive rate of 0.2%. The detection rate for other chromosomal abnormalities did not improve.
Adding the trisomy 18 algorithm to the Down screening program resulted in the detection of five additional trisomy 18 cases.