Frontal Anteroposterior Diameter and Derived Ratios Between 18 and 38 Weeks: Normative Curves for Fetal Neurosonography

Objective To establish gestational age‐specific reference curves for the frontal anteroposterior diameter (FAPD) and its derived ratios—FAPD to occipitofrontal diameter (FAPD/OFD), FAPD to head circumference (FAPD/HC), and FAPD to transverse cerebellar diameter (FAPD/TCD)—in structurally normal fetuses. Methods This was a retrospective, cross‐sectional study conducted in three fetal medicine centers in Brazil. Ultrasound images of 495 structurally normal singleton fetuses between 18 and 38 weeks of gestation were analyzed. FAPD was measured in the transventricular plane, and the FAPD/OFD, FAPD/HC, and FAPD/TCD ratios were calculated. Polynomial regression models were used to construct reference curves and percentiles. Correlations between biometric parameters and gestational age were assessed using Pearson or Spearman coefficients, as appropriate. Results A strong and statistically significant positive correlation was observed between FAPD and gestational age ( r = 0.93, p < 0.001; R 2 = 0.87), with an average increase of 1.215 mm per week. The FAPD/OFD and FAPD/HC ratios remained relatively stable across gestation, showing no significant correlation. In contrast, a moderate and significant inverse correlation was found between FAPD/TCD and gestational age ( r = −0.64, p < 0.0001; R 2 = 0.41). Reference values for the 5th, 50th, and 95th percentiles were established for all parameters throughout the studied gestational range. Conclusion The reference curves developed for FAPD and its derived ratios reflect normative frontal lobe growth patterns and provide a reliable framework for prenatal ultrasonographic assessment. The stability of FAPD/OFD and FAPD/HC, combined with the gestational decline in FAPD/TCD, underscores the importance of region‐specific brain measurements. These curves may aid in the early detection of atypical neurodevelopment and support future population‐specific research.