Measurement of nuchal skin fold thickness in the second trimester: influence of imaging angle and fetal presentation

Abstract

Objective: To assess the effect of imaging angle and fetal presentation on the measurement of nuchal skin fold thickness (NFT) in the second trimester.

Methods: Fetal NFT was prospectively measured in 921 women at 18-21 weeks' gestation. The population was divided into two groups according to fetal presentation. Group A comprised 643 fetuses in cephalic or transverse presentation and Group B comprised 278 fetuses in breech presentation. To determine whether there was a difference in NFT depending on imaging angle, in Group A the NFT was measured on the standard axial horizontal images and then on images obtained after moving the transducer about 30 degrees toward the occiput. In Group B an assessment was made as to whether NFT changed with fetal presentation. Initial measurements were made with the fetuses in breech presentation. Repeat NFT measurements were made after 15 min in 64 fetuses that had converted to cephalic or transverse presentation. Statistical analysis was performed using a paired t-test.

Results: The mean NFT on the standard horizontal image was 5.2 mm, while that on the 30 degrees occiput image was 4.0 mm. The difference was significant (P < 0.001). One hundred and twenty-three (19.1%) fetuses had a NFT greater than 6 mm on the standard image, but only two (0.3%) had an NFT of > 6 mm on the 30 degrees occiput image. There were no chromosomal or structural abnormalities in the 123 fetuses in which the NFT was > 6 mm. The mean NFT of 64 fetuses in the breech presentation was 4.4 mm, while it was 3.9 mm when they had converted to other presentations. The difference was statistically significant (P = 0.001). In breech presentation, 13/64 (20.3%) fetuses had a NFT greater than 6 mm compared to only one (1.6%) fetus in the other presentations. There were no structural or chromosomal abnormalities in any fetus with a NFT > 6 mm in either group.

Conclusions: In this study the NFT measurement varied with scan direction and fetal presentation. To avoid falsely thickened NFT measurements the scan should be directed to image the posterior surface of the occipital bone. This can be achieved by scanning from a 30 degrees occipital direction. Also, NFT is greater in fetuses in a breech presentation. Attention to these details can reduce false-positive NFT measurements.