Correlation between nuchal thickness and abnormal karyotype in first trimester fetuses.

Objective: To evaluate the accuracy of ultrasound measurement of nuchal thickness in first trimester fetuses for predicting fetal karyotype.

Design: A prospective study of the nuchal thickness of fetuses measured during an ultrasound examination in all women undergoing first trimester chorionic villus sampling (CVS).

Setting: Two major public hospitals and two associated private practices between 7 September 1993 and 6 September 1994.

Participants: Pregnant women with various indications for CVS (in 82% because of maternal age).

Results: 1306 women underwent CVS, including 11 with twin pregnancies: 1317 fetuses were tested. Karyotype results were obtained for 1312 fetuses: 41 (3.1%) had an abnormal karyotype, and 20 of these (49%) had a nuchal thickness measurement of 3 mm or more, compared with 44 (3.5%) of the 1271 fetuses with a normal karyotype. Of the 21 fetuses shown to have trisomy 21, 12 would have been detected if a nuchal thickness of 3 mm or more had been used as an indicator, giving a sensitivity of 57%. Nuchal thickness measurements of 1 or 2 mm excluded trisomy 21 with a negative predictive value of 99.3%. Fetuses with moderate nuchal thickening, normal karyotype and no other problems noted on the initial ultrasound scan had neonatal outcomes similar to those in the general obstetric population.

Conclusion: Nuchal thickening in the first trimester (10 weeks on) of pregnancy in a high risk population is a powerful indicator of increased risk of aneuploidy.