In patients with Down syndrome, an error occurs in the coming together of chromosome 21. The extra genetic material is responsible for the developmental abnormalities that occur. Instead of 46 chromosomes plus two sex chromosomes, there are 47. The most common error in chromosome replication is trisomy 21, where the new cell gets three copies of chromosome 21, instead of two. This accounts for about 95% of those patients with Down syndrome. Screening is a strategy used in a population to identify the possible presence of an as-yet-undiagnosed medical condition in apparently normal individuals who have no signs or symptoms. With down’s syndrome screening, the possibilities of trisomy 21 will estimated using various biochemical and ultrasound markers. Trisomic pregnancies are associated with altered maternal serum concentrations.
In trisomy 21 pregnancies, maternal serum free ß-hCG is about twice as high and PAPP-A is reduced to about half compared to chromosomally normal pregnancies.
Screening in the first trimester by a combination of maternal age, fetal NT, FHR and serum free ß-hCG and PAPP-A identifies about 90% of trisomy 21 pregnancies for a false positive rate of 3%.
Screening in the second trimester by maternal age and various combinations of total or free ß-hCG, AFP, uE3 and Inhibin A can identify 56-71% of trisomy 21 pregnancies for a false positive rate of 5%.