One-stop clinic for assessment of risk of chromosomal defects at 12 weeks of gestation

The first method of screening for trisomy 21, introduced in the early 1970s , was based on maternal age. Amniocentesis was offered to women aged 35 yea rs or more; this 'high-risk' group constituted 5% of the pregnant populatio n and contained 30% of trisomic pregnancies. In the late 1980s, a new metho d of screening was introduced that takes into account not only maternal age but also the concentration of various fetoplacental products (alpha-fetopr otein, estriol and human chorionic gonadotropin) in the maternal circulatio n at 16 weeks of gestation. This method of screening is more effective than maternal age alone and, for the same rate of invasive testing (5%), it can identify about 60% of the fetuses with trisomy 21. In the 1990s, screening by a combination of maternal age and fetal nuchal translucency thickness a t 11-14 weeks of gestation was introduced. This method has now been shown t o identify about 75% of affected fetuses for a screen-positive rate of 5%. When maternal serum free beta-hCG and pregnancy-associated plasma protein-A at 11-14 weeks are also taken into account, the detection rate of trisomy 21 and all major chromosomal defects is about 90%. Furthermore, the develop ment of new methods of biochemical testing, within 30 min of taking a blood sample, has now made it possible to combine ultrasound and biochemistry in one-stop clinics for assessment of risk (OSCAR).