In a study undertaken by investigators at the Harris Birthright Research Centre for Fetal Medicine, King's College Hospital, London and the Department of Clinical Biochemistry, Harold Wood Hospital, Essex, UK, over 30,000 first-trimester pregnancies were screened for trisomy 21 using a combined regimen of maternal age, fetal nuchal thickness ( NT ), maternal serum-free beta-human chorionic gonadotrophin ( ß-hCG ) and pregnancy-associated plasma protein-A ( PAPP-A ).

These tests were carried out in a One-Stop Clinic for first-trimester Assessment of Risk ( OSCAR ). This multidisciplinary center performed biochemical testing of the mother, ultrasound examination of the fetus, and post-test counseling, all within a 1-hour visit to the center.

The detection rate of trisomy 21 and other major chromosomal defects by this method was about 90%, for a false-positive rate of 5%.
This is far superior to the 30% detection rate achieved by maternal age.
Furthermore, with first-trimester combined screening, the 65% detection rate for trisomy 21 associated with second-trimester screening with the use of maternal serum biochemistry, can be achieved with a more than 10-fold reduction in the number of unnecessary invasive tests, from 5% to 0.4%.

Kypros H. Nicolaides, senior author, stated “ In our opinion, risk assessment for chromosomal defects in the first rather than the second trimester provides earlier reassurance for those with a normal result and less traumatic termination for those choosing this option.” Based on this extensive study, the authors believe that “assessment of risk by a combination of maternal age, fetal NT, and maternal serum-free b-hCG and PAPP-A, followed by selective termination of affected fetuses reduces the potential live birth prevalence of trisomy 21 by at least 90%."

Source: American Journal of Obstetrics and Gynecology, 2005


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