The effect of fetal neck position on nuchal fold thickness

OBJECTIVE: Our purpose was to determine whether ultrasonographic measuremen ts of nuchal fold thickness are affected by the position of the fetal neck. STUDY DESIGN: Fetal nuchal fold thickness was prospectively measured in 258 women undergoing routine ultrasonography at 15 to 21 completed weeks of ge station. Patients with fetal structural or chromosomal anomalies were exclu ded. At the time of examination the position of the fetal head was noted as being extended or flexed on the basis of the angle between the spine and t he base of the skull. Gestational age was based on menstrual dates or ultra sonographic biometric parameters. Data were tested for normality. Mann-Whit ney U test and analysis of covariance were used (significance was considere d to be P < .05). Data are presented as median and range. RESULTS: A total of 258 fetuses were examined with 167 (65%) in the flexed and 91 (35%) in the extended neck position. Gestational age was not signifi cantly different between the flexed and extended groups (median, 19.1 weeks ; range, 15.5-21.6 weeks; vs median, 19.1 weeks; range, 15.6-22 weeks; P=.2 3). Nuchal fold thickness was significantly lower in the flexed group than in the extended group (median, 3.5 mm; range, 1.3-6.2 mm; vs median, 3.9 mm : range, 2.2-4.9 mm; P=.0097). Nuchal fold thickness increased significantl y with gestational age in both groups. The difference in nuchal fold thickn ess between the 2 groups persisted even after the increase in nuchal fold t hickness was adjusted for with gestational age (P=.002, analysis of covaria nce). The difference between the 2 groups was higher at earlier gestations. CONCLUSION: Nuchal fold thickness is affected by gestational age and fetal neck position. Correction for these variables may improve the accuracy of n uchal fold thickness measurements in screening for fetal chromosomal anomal ies.