Prenatal magnetic resonance imaging differentiation between fetal subcutaneous fat thickening in macrosomia and subcutaneous edema: a description of two cases

Introduction

In cases of fetal macrosomia, distinguishing between increased subcutaneous fat and pathological edema is clinically important, but remains challenging using ultrasound (US). Fetal magnetic resonance imaging (MRI) may provide superior tissue characterization and anatomical localization, facilitating differentiation between these conditions (1-4). When US results are unclear and risk factors such as gestational diabetes are present, subcutaneous fat thickness may serve as a biomarker for the risk of macrosomia.

Fetal macrosomia, defined as a birth weight exceeding 4,000 g, is associated with an increased risk of shoulder dystocia, cesarean delivery, and postpartum hemorrhage (1). Fetal hydrops, defined as the accumulation of fluid in two or more fetal compartments (including ascites, pleural effusion, pericardial effusion, and generalized skin edema), has a poor prognosis, with an overall survival rate of only 9.2% in non-immune cases (5,6). Accurate distinction between these conditions is crucial, as misclassification may result in inappropriate clinical management.

Case presentation

All procedures in this study were performed in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Institutional Ethics Committee of Tangshan Women and Children’s Hospital (approval No. 2021-041-01). Written informed consent was obtained from the patients for the publication of this article and the accompanying images. Copies of the written consent forms are available for review by the editorial office of this journal.

All fetal MRI examinations were performed on a 1.5T Aera scanner (Siemens, Germany) using a body coil. The imaging protocol included T2‑weighted half‑Fourier acquisition single‑shot turbo spin‑echo (T2‑HASTE), true fast imaging with steady‑state free precession (TruFISP), T1‑weighted imaging (T1WI), and diffusion‑weighted imaging (DWI). The key parameters, including repetition time/echo time (TR/TE) and field of view (FOV), were as follows: T2‑HASTE, TR/TE, 1,100/97 ms, FOV, 310 mm, and slice thickness, 4.5 mm; TruFISP, TR/TE, 684.8/2.6 ms, FOV, 310 mm, and slice thickness, 4.5 mm; T1WI, TR/TE, 176/4.93 ms, FOV, 400 mm, and slice thickness, 7.0 mm; and DWI with b-values of 50 and 700 s/mm², TR/TE, 3400/76 ms, FOV, 200 mm, and slice thickness, 6.0 mm.

Case 1

A 33-year-old pregnant woman (gravida 5, para 2) with gestational diabetes mellitus (GDM) and obesity [body mass index (BMI), 36 kg/m²] was referred for fetal MRI at 37+4 weeks’ gestation. Prenatal US at 37+3 weeks’ gestation demonstrated polyhydramnios and mild diffuse thickening of fetal subcutaneous soft tissue (maximum thickness, 8.6 mm). Due to polyhydramnios, the patient was referred for a fetal abdominal MRI to exclude digestive tract malformations (e.g., esophageal atresia or duodenal atresia), which could affect fetal swallowing and result in polyhydramnios.

MRI showed diffuse thickening of the fetal subcutaneous fat, but no structural abnormalities were observed in the fetal gastrointestinal tract. The thickened subcutaneous fat demonstrated relatively high signal intensity on T1WI, moderately high signal intensity on T2-weighted imaging (T2WI), and signal suppression on fat-saturated images, with signal characteristics similar to those of maternal subcutaneous fat. The maximum thickness of the subcutaneous fat measured 14.5 mm (Figure 1).

Figure 1 Fetus at a gestational age of 37+4 weeks, with a birth weight of 4,500 g. The mother had gestational diabetes mellitus and obesity. (A) Coronal T2-HASTE image demonstrating marked thickening of the subcutaneous tissue of the chest and abdominal walls (white arrow), particularly involving the shoulders, back, and buttocks. The subcutaneous tissue shows intermediate-to-high signal intensity on T2-HASTE, which is lower than that of maternal subcutaneous fat (white arrowhead). (B) Sagittal T2-HASTE and sagittal fat-suppressed T2-HASTE images showing reduced signal intensity of fetal subcutaneous fat on the fat-suppressed image (yellow arrow) compared with the non-fat-suppressed image (white arrow). (C) Axial T2-HASTE image demonstrating not only thickening of the subcutaneous fat of the abdominal wall but also increased subcutaneous fat of the extremities (blue arrows, lower limbs). (D) Axial TruFISP axial image showing measurement of the maximum chest wall subcutaneous fat thickness (approximately 14.2 mm). The India ink artifact is visible at the fat-fluid interface (blue arrowhead) and fat-muscle interface (yellow arrowhead), consistent with the gradient-echo characteristics of the sequence. (E) Axial T1WI showing high signal intensity of fetal subcutaneous fat (white arrows). In the last image of panel E, the fetal subcutaneous fat of the abdominal wall (white arrow), extremities (blue arrows) and the maternal subcutaneous fat (white arrowhead) exhibit high signal intensity on T1WI. (F) ADC map of the fetal abdominal wall subcutaneous fat (orange star, adjacent to the maternal abdominal wall with minimal artifact) showing an ADC value of 1.9 × 10⁻³ mm²/s (region of interest size: 9.04 mm²). ADC, apparent diffusion coefficient; T1WI, T1-weighted imaging; T2-HASTE, T2-weighted half-Fourier acquisition single-shot turbo spin-echo; TruFISP, true fast imaging with steady-state free precession.

The patient underwent an elective cesarean section on the same day as the MRI examination (37+4 weeks’ gestation), primarily due to threatened labor in the context of GDM, polyhydramnios, and obesity. The MRI finding of increased subcutaneous fat thickness (14.5 mm) further supported the decision for cesarean delivery. A male neonate weighing 4,500 g was delivered, with a postnatal capillary blood glucose level of 5.3 mmol/L (normal range, 3.90~6.10 mmol/L).

Case 2

A 25-year-old pregnant woman (gravida 2, para 1) at 34+1 weeks’ gestation underwent fetal US, which detected bilateral pleural and peritoneal effusions, accompanied by cerebellar hypoplasia and a widened cisterna magna. The umbilical artery systolic/diastolic (S/D) ratio was 3.17, and the umbilical artery pulsatility index (PI) was 1.14, prompting a recommendation for fetal MRI.

At 35+1 weeks’ gestation, follow-up US results showed persistent effusions and reduced fetal cardiac function (S/D ratio, 3.57; PI: 1.30). The increased S/D ratio and PI values indicated fetal intrauterine hypoxia; a S/D ratio >3.0 or a PI >95th percentile for gestational age in the third trimester indicates compromised placental perfusion (7). Toxoplasmosis, others, rubella, cytomegalovirus, and herpes simplex (TORCH) testing was not performed.

Fetal MRI demonstrated findings consistent with fetal hydrops, including scalp and thoracoabdominal subcutaneous edema, pleural effusion, and ascites, along with cerebellar hypoplasia and an enlarged posterior fossa (approximately 11 mm) (Figure 2). The perinatal outcome was induced termination of pregnancy. Subcutaneous edema showed marked hyperintensity on T2WI, no signal attenuation on fat-suppressed images, hypointensity on T1WI, and increased signal intensity on the apparent diffusion coefficient (ADC) map, consistent with free fluid accumulation. Importantly, MRI enabled precise anatomical localization at the level of the scalp: subcutaneous edema was located beneath the aponeurosis within the loose areolar tissue layer, whereas subcutaneous fat thickening was confined to the region above the low-signal aponeurosis.

Figure 2 Fetus at a gestational age of 34+2 weeks. (A) Sagittal T2-HASTE image showing subcutaneous edema of the chest wall (white arrow), scalp edema located beneath the low-signal galea aponeurotica (yellow arrow), and pleural effusion (blue arrow). All edematous components demonstrate high signal intensity on T2WI. (B) Sagittal TruFISP image demonstrating ascites (orange arrow) with high signal intensity. (C) Coronal TruFISP image showing scalp edema (yellow arrow), chest wall edema (white arrow), and pleural effusion (blue arrow). (D) Axial TruFISP image demonstrating scalp edema (yellow arrow). (E) Axial T2-HASTE fat-suppressed image showing scalp edema (yellow arrow) without signal attenuation on fat suppression. (F) Axial T1WI demonstrating scalp edema with low signal intensity (yellow arrow) located beneath the high-signal subcutaneous fat layer. (G) ADC map showing high signal intensity in the region of the scalp edema (yellow arrow), with an ADC value of 3.1×10⁻³ mm²/s (region of interest size: 3.15 mm²). (H) Axial TruFISP image clearly showing the layers of the scalp. From outside to inside, the layers are: skin (low signal intensity), subcutaneous tissue containing fat (high signal intensity), galea aponeurotica (low signal intensity), subgaleal loose connective tissue containing subcutaneous edema (high signal intensity) intensity, and periosteum (low signal intensity), labelled as numbers 1–5 in the image. ADC, apparent diffusion coefficient; T1WI, T1-weighted imaging; T2-HASTE, T2-weighted half-Fourier acquisition single-shot turbo spin-echo; T2WI, T2-weighted imaging; TruFISP, true fast imaging with steady-state free precession.

Discussion

These case reports present the different MRI manifestations of fetal subcutaneous fat thickening and subcutaneous edema. US can detect true skin edema by identifying the hypoechoic line between the skin edge and the fetal bones or fascia (5). In contrast, multiparametric MRI clearly differentiates subcutaneous edema from subcutaneous fat based on their signal characteristics and provides precise anatomical localization: edema is located beneath the galea aponeurotica, while fat is located above it.

Fetal subcutaneous fat thickness is a reliable alternative indicator of macrosomia. Previous studies have reported a significant difference in the mean subcutaneous thickness between normal fetuses and macrosomic fetuses (7.0 vs. 12.4 mm), with abdominal subcutaneous fat thickness showing a strong positive correlation with birth weight (4). Anblagan et al. reported that fetal fat volume measured by MRI was higher in diabetic pregnancies and was also correlated with birth weight (8). In our macrosomic fetal case, the MRI measurement was 14.5 mm, exceeding both the US measurement (8.6 mm) and previously reported thresholds (4).

Recent MRI-based normative reference data have provided gestational-age-specific fetal volumetric curves that may help distinguish normal third-trimester adiposity from excessive fetal adiposity (9). On fetal T1WI, the subcutaneous fat showed high signal intensity comparable to that of maternal fat, consistent with the characteristic of higher fat maturation, while the lower signal intensity on T2WI may reflect differences in composition between fetal and adult fat (10,11). In addition, the Dixon technique uses differences in the resonance frequencies of fat and water to accurately calculate fat tissue volume and fat fraction, enabling effective differentiation between water and fat in subcutaneous tissue. More importantly, it can be applied to MRI-based fetal body composition growth charts, including fat mass and fat-free mass (12), which are very valuable for detecting excessive fetal growth, enabling early risk stratification, and guiding targeted interventions.

Given that fetal edema is associated with a high perinatal mortality rate, accurate diagnosis is very important (13). A modern diagnostic algorithm outlines the current evaluation and management of non-immune hydrops fetalis, including fetal echocardiography, karyotype analysis, and infectious testing (14). This report has several limitations: it includes only two cases, lacks postpartum histopathological confirmation, and the case of edema did not undergo a complete etiological investigation.

Conclusions

Multiparametric fetal MRI can accurately distinguish physiological fat accumulation from fetal edema, improving diagnostic confidence and optimizing prenatal management.

Acknowledgments

We sincerely thank the patients for providing their data for analysis.

Footnote

Funding: This study was funded by the 2022 Hebei Provincial Medical Science Research Program (project No. 20221762).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2026-1194/coif). All authors report that this study was funded by the 2022 Hebei Provincial Medical Science Research Program (project No. 20221762). The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the institutional ethics committee of Tangshan Women and Children’s Hospital (approval No. 2021-041-01). Written informed consent was obtained from the patients for the publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. Badr DA, Carlin A, Boulvain M, Kadji C, Cannie MM, Jani JC, Gucciardo L. A simulation study to assess the potential benefits of MRI-based fetal weight estimation as a second-line test for suspected macrosomia. Eur J Obstet Gynecol Reprod Biol 2024;297:126-31. [Crossref] [PubMed]
2. Badr DA, Cannie MM, Kadji C, Kang X, Carlin A, Jani JC. Reducing macrosomia-related birth complications in primigravid women: ultrasound- and magnetic resonance imaging-based models. Am J Obstet Gynecol 2024;230:557.e1-8.
3. Kadji C, Cannie MM, Kang X, Carlin A, Benjou Etchoua S, Resta S, Dütemeyer V, Abi-Khalil F, Mazzone E, Bevilacqua E, Jani JC. Fetal magnetic resonance imaging at 36 weeks predicts neonatal macrosomia: the PREMACRO study. Am J Obstet Gynecol 2022;226:238.e1-238.e12. [Crossref] [PubMed]
4. Mazzone E, Kadji C, Cannie MM, Badr DA, Jani JC. Prediction of large-for-gestational age at 36 weeks' gestation: two-dimensional ultrasound vs three-dimensional ultrasound vs magnetic resonance imaging. Ultrasound Obstet Gynecol 2024;63:489-96. [Crossref] [PubMed]
5. Dalton SE, Griffith AM, Kennedy AM, Woodward PJ. Differential Diagnosis of Hydrops Fetalis: An Imaging Guide. Radiographics 2025;45:e240158. [Crossref] [PubMed]
6. Gowda M, Gupta A, Shreyanka HV, Avardal T, Aishwarya TP, Murugesan A, Khare C. Evaluation of Non-Immune Fetal Hydrops in Resource Poor Country: Challenges Faced in Ascertaining the Etiology. Prenat Diagn 2025;45:939-50. [Crossref] [PubMed]
7. Martins JG, Biggio JR, Abuhamad A. Society for Maternal-Fetal Medicine Consult Series #52: Diagnosis and management of fetal growth restriction: (Replaces Clinical Guideline Number 3, April 2012). Am J Obstet Gynecol 2020;223:B2-B17.
8. Anblagan D, Deshpande R, Jones NW, Costigan C, Bugg G, Raine-Fenning N, Gowland PA, Mansell P. Measurement of fetal fat in utero in normal and diabetic pregnancies using magnetic resonance imaging. Ultrasound Obstet Gynecol 2013;42:335-40. [Crossref] [PubMed]
9. Bouachba A, Bartin R, De Jesus Neves J, Bussières L, Grévent D, Virfollet J, Gauchard G, Bobet L, Roux N, Glemain B, Salomon LJ, Gorincour G. Normative range of MRI-based fetal body volume and association with ultrasonographically estimated fetal weight at 16-36 weeks: prospective study. Ultrasound Obstet Gynecol 2025;66:81-8. [Crossref] [PubMed]
10. Blondiaux E, Chougar L, Gelot A, Valence S, Audureau E, Ducou le Pointe H, Jouannic JM, Dhombres F, Garel C. Developmental patterns of fetal fat and corresponding signal on T1-weighted magnetic resonance imaging. Pediatr Radiol 2018;48:317-24. [Crossref] [PubMed]
11. Giza SA, Miller MR, Parthasarathy P, de Vrijer B, McKenzie CA. Comparison of modified two-point dixon and chemical shift encoded MRI water-fat separation methods for fetal fat quantification. J Magn Reson Imaging 2018;48:274-82. [Crossref] [PubMed]
12. Rabinowich A, Avisdris N, Yehuda B, Vanetik S, Khawaja J, Graziani T, Neeman B, Wexler Y, Specktor-Fadida B, Herzlich J, Joskowicz L, Krajden Haratz K, Hiersch L, Ben Sira L, Ben Bashat D. Fetal body composition reference charts and sexual dimorphism using magnetic resonance imaging. Am J Clin Nutr 2024;120:1364-72. [Crossref] [PubMed]
13. Dunn SB, Whittington JR. Nonimmune Hydrops Fetalis. Neoreviews 2024;25:e475-85. [Crossref] [PubMed]
14. Society for Maternal-Fetal Medicine (SMFM). Sparks TN, Norton ME, SMFM Publications Committee. Society for Maternal-Fetal Medicine Consult Series #75: Evaluation and management of non-immune hydrops fetalis. Pregnancy 2026;2:e70208.