OC38.03: One stop clinic for assessment of risk (OSCAR) for trisomy 21 in singleton pregnancies at 11 to 13 + 6 weeks of gestation

Abstract

Objective: To evaluate the performance of first-trimester screening for trisomy 21 using maternal age, fetal nuchal translucency (NT) and maternal serum free s-hCG and PAPP-A at 11 to 13 + 6 weeks. Methods: All women attending between 11 to 13 + 6 weeks of gestation were offered screening for trisomy 21. Assessment of NT was performed by appropriately trained sonographers (Fetal Medicine Foundation Certificate of competence), and maternal serum s-hCG and PAPP-A was measured using the Kryptor analyser. Patients were counselled with regards to their combined estimated risk and the available options for the management of the pregnancy. Data on pregnancy outcome were obtained from the cytogenetics laboratory, and by letters and telephone calls to the patients, their general practitioners or the maternity units in which they delivered. Detection rates and false positive rates were calculated for use of maternal age, maternal age and NT, and maternal age, fetal NT and maternal biochemistry. Results: Screening for trisomy 21 was carried out in 31 904 singleton pregnancies and pregnancy outcome was obtained in 30 564 (95.8%) cases. There were 330 pregnancies affected with a chromosomal abnormality, including 196 cases of trisomy 21. Of these, in 185 cases the diagnosis was made prenatally (178 in the screen positive and seven in the screen negative pregnancies) and 11 where the diagnosis was made postnatally in live births (five in the screen positive pregnancies, in which the parents chose to avoid invasive testing, and six in the screen negative pregnancies). The estimated risk for trisomy 21 was 1 in 300 or greater in 7.5% of the normal pregnancies, in 93.4% of those with trisomy 21 and in 88.8% of those with other chromosomal defects. Conclusion: Currently, the most effective method of screening for trisomy 21 is by a combination of maternal age, fetal NT and maternal serum free s-hCG and PAPP-A at 11 to 13 + 6 weeks.