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The First World Congress On:
Controversies in Obstetrics, Gynecology & Infertility |
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Nuchal Translucency Screening
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Nuchal translucency is the area at the back of the fetal neck, which can be measured at 10-14 weeks gestation. It is the black area lying between the subcutaneous fascia and skin. It increases with increasing gestational age, and great care must be taken to measure it correctly to avoid measurement error. Several groups have demonstrated an association between an increased nuchal translucency and aneuploidy, particularly Down syndrome. There are many studies that have shown that nuchal translucency measurement can be used as a screening test for Down syndrome between 10-14 weeks gestation. Detection rates vary between around 40% and 70% in both selected and unselected populations (Chitty and Pandya 1997). In the largest study to be published from the King’s group, who have pioneered this research, 82% of Down syndrome fetuses were detected in over 96,000 pregnancies screened in 22 centres by one of 306 sonographers for a 8.3% false positive rate (Schnijders at al 1998). This figure fell to 77% if the false positive rate was set at 5%. When corrected for the prevalence of Down syndrome at different gestational ages, detection rates using nuchal translucency are equivalent to those achieved in programmes using second trimester serum screening (Haddow 1998). In order to improve on this significantly it is proving necessary to include measurement of first trimester maternal serum analytes (free beta human chorionic gonadotrophin and pregnancy associated protein-A) (Wald and Hackshaw 1997; de Graaf et al 1999). In addition to the association with aneuploidy, and an increased nuchal translucency has been found to be a marker of other underlying pathologies (Chitty and Pandya 1997). These include genetic syndromes (Brady et al 1998), but the most common association appears to be with cardiac abnormalities (Hyett et al 1998). It seems clear that nuchal translucency measurement may be useful in screening for a variety of fetal abnormalities. However, whilst there may be advantages in early detection of anomalies there are several disadvantages which should be considered (Chitty and Panda 1997). Many pregnancies complicated by fetal abnormality, both aneuploidy and other anomalies, will end in an early miscarriage. If these pregnancies are identified early parents may be asked to make difficult decisions regarding termination of pregnancy, thereby imposing a potential burden and long-term consequences which may could have been avoided had the pregnancy been lost spontaneously. In addition, having identified an increased risk of aneuploidy it is only reasonable to offer early karyotyping, which necessitates chorionic villus sampling (CVS) in early pregnancy as early amniocentesis has been shown to be associated with increased risks. CVS may carry a slightly increased risk of procedural related miscarriage, has been shown to have 1-2% risk of mosaicism which may present a diagnostic dilemma. Furthermore, karyotyping using chorionic villi is more expensive than using second trimester amniotic fluid. Many consider that screening using nuchal translucency in the first trimester requires further evaluation to assess the natural history, clinical cost effectiveness and parental psychological aspects before widespread implementation as a general screening test to be applied to the general population, although its use in high risk women seems clear. References Brady, A.F., Pandya, P.P., Yuksel, B., Greenough, A., Patton, M.A., Nicolaides, K.H. Outcome of chromosomally normal livebirths with increased fetal nuchal translucency at 10-14 weeks gestation, J Med Genet, 1998, 35, 222-224. Chitty L S, Pandya P. Ultrasound screening for fetal abnormalities in the first trimester. Prenatal Diagnosis, 1997, 17:1269-1282 Chitty L S. Antenatal screening for chromosomal abnormalities. Br Med Bull, 1998;54:839-856. de Graaf, I.M., Pajkrt, E., Bilardo, C.M., Lescho, N.J., Cuckle, H.S., van Lith, J.M.M. Early Pregnancy Screening for Fetal Aneuploidy with Serum Markers and Nuchal Translucency. Prenat. Diagn. 1999, 19;458-462. Haddow JE. Antenatal screening for Down’s syndrome: where are we and where next? Lancet, 1998, 352:336-7. Hyett J, Perdu M, Sharland G, Snijders R, Nicolaides KH. Using fetal nuchal translucency to screen for major congenital cardiac defects at 10-14 weeks of gestation: Population based cohort. Br Med J, 1999,318:81-84. Snijders RJM, Noble P, Sebire N, Souka A, Nicolaides KH. UK multicentre project on assessment of risk of trisomy 21 by maternal age and fetal nuchal translucency thickness at 10-14 weeks of gestation. Lancet, 1998:343-346. Wald NJ, Hackshaw AK. Combining ultrasound and biochemistry in first trimester screening for Down’s syndrome. Prenatal Diagnosis, 1997, 821-829. |
