Objective: Our purpose was to determine whether ultrasonographic measurements 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 gestation. Patients with fetal structural or chromosomal anomalies were excluded. 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 the base of the skull. Gestational age was based on menstrual dates or ultrasonographic biometric parameters. Data were tested for normality. Mann-Whitney U test and analysis of covariance were used (significance was considered 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 significantly 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 = .23). 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 significantly with gestational age in both groups. The difference in nuchal fold thickness between the 2 groups persisted even after the increase in nuchal fold thickness was adjusted for with gestational age (P = .002, analysis of covariance). 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 nuchal fold thickness measurements in screening for fetal chromosomal anomalies. (Am J Obstet Gynecol 2000;183:995-7.)