Routine Measurement of Nuchal Thickness in the Second Trimester

Abstract

Nuchal thickness was measured prospectively in all fetuses of 15 0/7 to 20 6/7 weeks scanned from January 1, 1988, to October 31, 1990. The most frequent indication for scan was gestational age. Of the 7,106 scans performed, 30 fetuses had an abnormal thickness of 6 mm or more. Twelve of the 30 had an abnormal karyotype: 9 with trisomy 21, 1 with triploidy and tetrasomy 12, 1 with partial trisomy 9, and 1 with trisomy 13 plus a 3,9 translocation. Two fetuses with normal karyotypes had another reason for nuchal thickening—Noonan's syndrome in one and achondrogenesis in the other. Seven of the 18 fetuses who had a normal karyotype had an abnormal thickness measured at the upper end of the gestational age range between 20 0/7 and 20 6/7 weeks. Increased nuchal thickness is infrequently (0.4%) detected in the early second trimester and is associated with abnormal karyotypes (40%) and other anatomic findings. A nuchal thickness measurement should be part of every scan performed between 15 0/7 and 19 6/7 weeks, inclusive. When thickness is abnormal, a detailed scan is indicated and karyotype analysis should be offered.

Key Words:

• Karyotype
• Ultrasound
• Down's syndrome
• Trisomy