Evaluation of fetal frontal lobe development and Evans index in pregnancies complicated by gestational diabetes mellitus: prospective comparative observational study.

OBJECTIVE: To assess whether third-trimester fetal frontal lobe biometry and ventricular-cranial proportionality differ between pregnancies with gestational diabetes mellitus (GDM) and normoglycemic controls, and whether these markers vary by treatment (diet vs. insulin) or composite adverse perinatal outcome (CAPO). METHODS: In this single-center prospective observational study, 73 singleton GDM pregnancies and 73 low-risk controls underwent ultrasound at comparable gestational ages (30-38 weeks). On the transventricular plane we measured frontal anteroposterior diameter (FAPD) and occipitofrontal diameter (OFD), and derived two indices (100×FAPD/OFD; 100×FAPD/head circumference [HC]). Maximal width of the frontal horns (MWFH) and maximal inner cranial diameter (MID) were recorded and the fetal Evans index (EI, MWFH/MID) calculated. Obstetric and neonatal outcomes were compared, with subgroup analyses within the GDM cohort. Multiple linear regression adjusted the main neurosonographic outcomes for estimated fetal weight, gestational age at ultrasound, maternal age, BMI, and parity. RESULTS: Gestational age at scan was similar (33.8 ± 1.9 vs. 33.6 ± 2.0 weeks), while estimated fetal weight was higher in GDM (2504 ± 514 vs. 2320 ± 531 g; p = 0.035). FAPD was modestly higher in GDM (median 39.4 vs. 38.9 mm; p = 0.040) with comparable OFD, and both frontal indices were higher (FAPD/OFD 37.61 vs. 36.59, p < 0.001; FAPD/HC 12.74 vs. 12.55, p < 0.001). MWFH and MID were slightly larger (p = 0.025 and 0.006), yet EI was unchanged (0.263 ± 0.011 vs. 0.262 ± 0.014; p = 0.574). After adjustment, GDM remained linked to a lower OFD (- 2.516 mm; 95% CI - 3.730 to - 1.303; p < 0.001), a higher FAPD/OFD ratio (adjusted mean difference 1.088; 95% CI 0.633-1.542; p < 0.001) and FAPD/HC ratio (0.150; 95% CI 0.033-0.268; p = 0.012), but not to FAPD, MWFH, MID, or EI. Major neonatal morbidity, NICU admission, and CAPO rates were comparable. Within the GDM group, neurosonographic measures did not differ by diet vs. insulin, and CAPO tracked with lower birth weight, not EI or frontal indices. CONCLUSION: GDM showed small but significant shifts in proportional fetal frontal indices that held after adjustment for fetal size and maternal covariates, whereas absolute FAPD, ventricular measurements, and EI showed no independent association. Because OFD was lower after adjustment while FAPD was not, the higher ratios partly reflect their denominator and cannot be read as greater absolute frontal growth. Frontal proportions appear altered while ventricular-cranial proportionality is preserved, though the clinical and neurodevelopmental meaning is still unclear. Longitudinal work using objective glycemic metrics and standardized postnatal neurodevelopmental follow-up is needed.