Nomograms of the axial transverse diameter of the fetal foramen magnum between 14 and 40 weeks' gestation

OBJECTIVE The purpose of this study was to create reference range nomograms of the axial transverse diameter of the fetal foramen magnum (TDFM) between 14 and 40 weeks' gestation. METHODS This cross-sectional study included pregnant patients between 14 and 41 weeks' gestation. Inclusion criteria consisted of well-established dates (confirmed by early sonography) and nonanomalous singleton fetuses with intact amniotic membranes. Sonographic measurements included biparietal diameter, head circumference, abdominal circumference, femur length, humerus length, transcerebellar diameter, and sonographically estimated fetal weight. Values of the TDFM were each calculated as the mean of 3 separate measurements. The 5th, 50th, and 95th percentiles were estimated at each week of gestational age (GA) by least squares regression for the mean and SD of the TDFM as functions of GA. R(2) and associated P values for the relationships between the TDFM and other biometric measurements were calculated. RESULTS The study included 602 consecutive patients meeting the inclusion criteria. The mean maternal age +/- SD was 27.5 +/- 6.4 years; median gravidity, 3 (range, 1-12); and median parity, 1 (range, 0-7). The mean TDFM was 13.5 +/- 3.7 mm. The reference curve formulas for the mean and SD of the TDFM (mm) were TDFM = -12.58 + GA/0.5616 - GA(2)/24.9 + GA(3)/2430.7, and SD(TDFM) = 1.05 - GA/19.4 + GA(2)/588.8. The TDFM correlated significantly and strongly with biparietal diameter, head circumference, abdominal circumference, humerus length, femur length, transcerebellar diameter, and estimated fetal weight (all R(2) >or= 0.90; all P < .0001). CONCLUSIONS We present reference range nomograms of the TDFM between 14 and 40 weeks' gestation.