The Normal Fetal Cephalic Index in the Second and Third Trimesters of Pregnancy

Abstract

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.

Sonographic indicators of isolated fetal sagittal craniosynostosis during pregnancy

“…4, which shows a scatter plot of the change in CI versus time difference between scans for both groups, including the least-squares regression lines. From this plot, it is evident that the change in CI in sagittal synostosis fetuses is more likely to be negative compared with the normal population 12 ; that is, the head shape is more likely to become more scaphocephalic than the normal population which tends to become slightly more brachycephalic, although this effect tends to reduce the further the scans are apart. In addition, there were two potential outliers, where a relatively large positive change was observed given the time difference between scan (both of these had scans that were separated by 12 weeks or less).…”

“…The data obtained were compared with the local population data obtained from the Pregnancy Outcomes Unit, SA Health (Pregnancy Outcomes in South Australia 2003–2016) 14 , the Northern Territory Government, Department of Health (NT Midwives’ Collection, Mothers and Babies 2003–2015) 15 and the Fetal Cephalic Index Study 12 …”

“…The change in CI between growth and morphology scans was calculated for normal fetuses 12 and those with sagittal synostosis. The difference in average CI between the two groups was assessed at each scan using Welch’s t ‐test, as was the average change in CI.…”

“…A progressive reduction in fetal cephalic index (CI) and deflection in the biparietal diameter (BPD) curve have been described previously as indicators of sagittal synostosis 5,6 . The normal fetal cephalic index in the second half of pregnancy has recently been determined 12 . This information may assist in the recognition of unusually scaphocephalic fetal skulls in those undergoing scanning in the third trimester.…”

“…5,6 The normal fetal cephalic index in the second half of pregnancy has recently been determined. 12 This information may assist in the recognition of unusually scaphocephalic fetal skulls in those undergoing scanning in the third trimester.…”