Fetal density evolution and association with ultrasonographically estimated fetal weight: prospective study

To assess fetal density throughout gestation using magnetic resonance imaging (MRI) and its potential application in early fetal weight estimation.

This single-center prospective study included healthy pregnant women with a singleton fetus at 16-36 weeks' gestation evaluated between December 2021 and June 2022, and was conducted as part of the 'Lumiere on the Fetus' trial. Fetal density was calculated as the ratio between ultrasound-based estimated fetal weight and the raw fetal body volume (FBV) measured using MRI. Measurements were performed from 16 to 36 weeks. Mean fetal density values were recorded across gestational age. A third-degree polynomial regression was fitted to model the non-linear relationship between gestational age and fetal density, and the results were analyzed accordingly.

In total, 260 healthy pregnant women with a singleton fetus were recruited, of whom 196 cases with ultrasound examination performed on the same day or within 7 days before the MRI scan were included in the analysis. Fetal density declined sharply from the early to mid second trimester, decreasing from a mean of 1.42 g/cm³ at 16 weeks to 1.12 g/cm³ at 20 weeks, and then stabilizing at a mean of 1.02 g/cm³ after 22 weeks. A third-degree polynomial regression equation (fetal density = (-1.36 × 10^-4 × GA³) + (0.0126 × GA²) - (0.390 × GA) + 5.01, where GA is gestational age in weeks) was fitted to model this non-linear relationship, highlighting a physiological transition in fetal density at approximately 22 weeks' gestation that may be linked to early changes in tissue composition.

Fetal density exhibited a sharp decline from 16 to 20 weeks before stabilizing at a mean of 1.0 g/cm³ after 22 weeks, enabling FBV to approximate fetal weight in g beyond this point based on the principle that weight = volume × density. MRI-based fetal density could serve as a valuable parameter for detecting growth abnormalities in early pregnancy and refining fetal-weight prediction models. © 2025 International Society of Ultrasound in Obstetrics and Gynecology.