The cephalic index (CI) is used in the evaluation of individuals with craniosynostosis. There is little agreement as to the normal range and stability of the CI during the fetal period, partly due to limited literature. We sought to determine the range, distribution and stability of the fetal CI in the second half of pregnancy. We also aimed to identify any relationship to delivery complications such as obstructed labor and malpresentation.The fetal head circumference, biparietal diameter (BPD) and occipitofrontal diameter (OFD) measurements were obtained from standard ultrasound images. Each of 4304 fetuses had measurements taken at morphology scan performed between 17 and 22 weeks' gestation, and at growth scanning at 28 to 33 weeks' gestation. The cephalic index was calculated using the formula: CI = BPD/OFD × 100. The distribution of the CI at both scans is very close to a normal distribution. The mean CI at 17 to 22 weeks was 75.9 (SD, 3.7); the mean CI at 28 to 33 weeks was 77.8 (SD, 3.5). The mean change in CI was 1.9 (SD, 4.28), which is not statistically significantly different from zero (t = 0.656, P = 0.512, 95% confidence interval). No relationship was found between the CI in normal fetuses and delivery complications. There is a wide variation in the change in CI in the third trimester. A value below the normal range in the third trimester or a progressive reduction in CI during the latter half of pregnancy should provoke detailed scanning of the fetal cranial sutures to check for craniosynostosis.