Normative range for MRI amniotic fluid volume from 16 to 36 gestational weeks: Absolute and relative values, and...

Amniotic fluid volume (AFV) is clinically important, yet ultrasound-based indices correlate poorly with true volume. MRI provides accurate volumetry, but normative reference data for AFV remain scarce. This study aimed to establish MRI-based gestational-age reference ranges for AFV and the AFV-to-fetal body volume ratio (AFV/FBV) from 16 to 36 weeks' gestation and to compare ultrasound amniotic fluid index (AFI) with MRI-derived AFV.

Prospective observational cohort study conducted at a single tertiary fetal imaging center (LUMIERE, Necker Hospital, Paris, France). Healthy singleton pregnancies between 16 and 36 gestational weeks were included; pregnancies with fetal anomalies or maternal complications were excluded. T2-weighted MRI with manual segmentation was performed to quantify AFV and fetal body volume. In a predefined subset, AFI was measured on same-day ultrasound. The main outcomes were AFV, AFV/FBV ratio, and their gestational-age normative ranges. The secondary outcome was the correlation between AFI and MRI-derived AFV. Percentile curves were derived using quadratic polynomial modeling.

ClinicalTrials.gov NCT04142606.

322 pregnancies were included (median gestational age 27.9 weeks; 45% male). AFV increased with gestational age (β = 6.61), peaking around 31 weeks, whereas AFV/FBV ratio declined (β = -0.08). No sex differences were observed. Weekly distributions were normally distributed. Quadratic polynomial modeling yielded physiologically plausible percentile curves (R² = 0.74 for AFV/FBV). In the ultrasound-MRI subset (n = 103), AFI significantly correlated with AFV (Spearman ρ = 0.77), although with wide individual variability.

This study provides the first MRI-based normative charts for AFV and AFV/FBV ratio between 16 and 36 weeks' gestation. The AFV/FBV ratio represents a novel physiological marker integrating fetal growth and fluid balance. These reference ranges may support future research and clinical decision-making.