SEARCH

SEARCH BY CITATION

Keywords:

  • fat mass;
  • lean mass;
  • ultrasound

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Objective

To determine reference values of fetal subcutaneous tissue thickness (SCTT) throughout gestation in a healthy population and to compare them with those from a population of pregnant women with gestational diabetes under standard therapy.

Methods

Three hundred and three women recruited from a high-risk pregnancy clinic were classified as being healthy (n = 218) or as having gestational diabetes (n = 85) on the basis of a negative or positive oral glucose tolerance test, respectively. They were enrolled into the cross-sectional study at 20 weeks' gestation. Ultrasound examinations were performed approximately every 3 weeks until delivery at term. The mid-arm fat mass and lean mass (MAFM, MALM), the mid-thigh fat mass and lean mass (MTFM, MTLM), the abdominal fat mass (AFM) and the subscapular fat mass (SSFM) were evaluated. Time-specific reference ranges were constructed from the 218 healthy women and a conventional Student's t-test was performed to compare SCTT values between the two study groups throughout gestation.

Results

Normal ranges, including 5th, 50th and 95th centiles of the distribution, were generated for each SCTT parameter obtained in each of the two groups of women. Significant differences were found between the two study groups at 37–40 weeks' gestation for MTFM, at 20–22 and 26–28 weeks for MTLM, at 31–34 and 35–37 weeks for MAFM, at 26–28 and 38–40 weeks for SSFM, and at 39–40 weeks for AFM, the mean residual values always being greater in gestational diabetic women than they were in the group of healthy pregnant women.

Conclusions

We provide gestational age-specific reference values for fetal SCTT. Fetal fat mass values, particularly in late gestation, are greater in women with gestational diabetes compared with healthy women. The reference values may have a role in assessing the influence of maternal metabolic control on fetal state. Copyright © 2003 ISUOG. Published by John Wiley & Sons, Ltd.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Fetal weight is commonly estimated using ultrasound-derived anthropometric measurements and population-based growth charts. The abdominal circumference is the most sensitive among the individual fetal parameters for the detection of fetal over-growth1. Nevertheless, both estimated fetal weight and abdominal circumference show a wide range of error (± 10%) that could impact on clinical practice2, 3.

Fat content correlates directly with energy stores and fat and lean body mass are often used in the nutritional assessment of an individual. Fat constitutes 12–14% of normal birth weight yet has been demonstrated to account for 46% of its variance4. As such, ultrasound-generated estimates of fetal fat may be useful in the evaluation of fetal growth abnormalities.

Ultrasound-derived anthropometric measurements of fetal body composition have previously been obtained. Bernstein et al.5 compared fat and lean body mass measurements in healthy fetuses across gestation and showed significant correlations with both birth weight and estimates of neonatal lean and fat mass. Galan et al.6 reported that the reduced birth weight of a subset of North American newborns was the result of a reduction in fetal subcutaneous fat tissue and not in lean mass, highlighting the potential usefulness of longitudinal fetal subcutaneous ultrasound measurements for detection of differences in specific populations.

Gestational diabetes (GD) mellitus is the commonest metabolic disorder of pregnancy and is defined as ‘varying degrees of glucose intolerance first recognized in pregnancy’7. The obstetric complications of GD are linked to the vaginal delivery of a large-for-gestational age fetus8 and to the increased risk of late stillbirth9. The neonatal metabolic complications of GD, including hypoglycemia and hypocalcemia, are caused by fetal hyperinsulinemia which occurs as a consequence of maternal hyperglycemia10. In GD patients maternal glycemia and obesity appear to be independent contributors to the occurrence of fetal macrosomia, operative delivery, hypertension and thromboembolic disease11. Subsequently, GD appears to be associated with the development of diabetes in the mother and diabetes and obesity in the offspring12.

There are conflicting guidelines for the management of GD patients, but there is agreement on the efficacy of dietary regimen and insulin therapy to reduce fetal size, and in particular fetal adiposity13. For these reasons we considered that the availability of reference values for fetal fat tissue measurements may be useful for clinical practice, in particular to identify excessive fetal fat deposition in normal and GD pregnancies.

The aims of this study were to construct reference ranges of fetal subcutaneous tissue thickness (SCTT) in a selected population of normal pregnancies, as determined by oral glucose tolerance test, and to compare these values with those from pregnant women with GD.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Patient selection

This was a cross-sectional study in which high-risk patients at 20–22 weeks' gestation from our outpatient clinic were enrolled between January and December 2001. Considered as risk factors were: family history of diabetes mellitus (first-degree relative), body mass index ≥ 27 kg/m2, glycosuria > 600 mg/L, previous delivery of a baby with birth weight ≥ 4500 g, previous GD, age ≥ 37 years, or polyhydramnios. Inclusion criteria were: singleton pregnancy, certain gestational age, and absence of fetal anomalies. Patients with twin pregnancies, Type I or II diabetes, chronic hypertension and fetal growth restriction were excluded. A total of 325 patients (180 in Rome, 145 in Florence) were initially recruited. Twenty-two were subsequently excluded, 19 for incomplete perinatal data and three due to the appearance of reduced fetal growth and oligohydramnios. The remaining 303 consisted of 218 healthy pregnant women on the basis of the oral glucose tolerance test, and 85 (28%) with GD.

At 24–28 weeks' gestation patients underwent the ‘one-step approach’ for the diagnosis of GD14 and were given a 100-g oral glucose tolerance test according to the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus14 and the results were evaluated according to the National Diabetes Data Group's criteria15. On the basis of the glucose tolerance test the patients were classified as ‘being healthy’ (this group was used for construction of the normal reference ranges) or as ‘having GD’.

Those meeting the criteria for the diagnosis of GD were started on a standard 1600-Kcal diet (50% carbohydrates). After 7–10 days of the dietary regimen, home monitoring of blood glucose was undertaken, assessing the peripheral blood glucose level with a commercially available glucose testing device (OneTouch Ultra, Lifescan, Milpitas, CA, USA) for a week to evaluate the efficacy of the therapy. The therapeutic goals were: preprandial blood glucose level ≤ 90 mg/dL and 2 h postprandial level ≤ 120 mg/dL16. When the daily pre- and postprandial blood glucose values were frequently > 100 mg/dL and > 120 mg/dL, respectively, short-term insulin therapy was added to the diet (Actrapid, Novo Nordisk, Bagsvaerd, Denmark) before breakfast, lunch and dinner17. Insulin doses were subsequently adjusted individually after a second blood glucose profile17. Eighteen of 85 (21.2%) patients had subcutaneous insulin treatment to re-establish the optimal levels of glycemia.

Sonography

Serial ultrasound examinations were performed approximately every 3 weeks until delivery at term. At each examination the patients underwent a detailed ultrasound scan with a commercially available Tecknos Esaote (Genova, Italy) ultrasound machine equipped with a 3.5- or 5-MHz probe (at the Fatebenefratelli Hospital, Tor Vergata University, Rome), or with an ATL HDI 5000 ultrasound machine (ATL, Seattle, WA, USA) with a 3.5-MHz transducer (at the Department of Gynecology, Perinatology and Human Reproduction, University of Florence, Florence).

Routine sonographic biometric parameters measured included head and abdominal circumference, and femur and humerus length. To obtain fat mass and lean mass, several measurements were assessed. We used the technique of Bernstein et al.5 to measure the fat and lean body mass areas on axial ultrasound images of the mid upper arm and mid upper leg, and on the cross-sectional images of the abdomen and the subscapular field5, 18, 19. Briefly, mid-arm fat mass (MAFM, cm2), mid-arm lean mass (MALM, cm2), mid-thigh fat mass (MTFM, cm2) and mid-thigh lean mass (MTLM, cm2) were obtained as follows (Figure 1): a sagittal view of the long bone and extremity was obtained in the middle of the ultrasound screen at an angle of 0° to the transducer. The transducer was then rotated 90° in the middle of the long bone to obtain the axial view of the extremity. The fat mass (MAFM, MTFM) was measured by taking the total cross-sectional limb area and subtracting the central lean area (MALM, MTLM) that consisted of muscle and bone. The abdominal fat mass (AFM, mm) was determined by measuring the thickness of the anterior abdominal subcutaneous tissue on the same axial image as that used for abdominal circumference measurement (Figure 1), as previously reported by Gardeil et al.20. Subscapular fat mass (SSFM, mm) was evaluated by taking the shoulder skin width perpendicularly to the bone at its lower end.

thumbnail image

Figure 1. Ultrasound images (a, c, e) and schematic representations (b, d, f) showing how the different subcutaneous tissue thickness measurements are obtained. (a,b) Axial view of the fetal arm showing mid-arm fat mass (MAFM) and mid-arm lean mass (MALM) evaluation; the process is similar for mid-thigh fat and lean mass. (c,d) Evaluation of the subscapular fat mass (SSFM) measurement. (e,f) Evaluation of the abdominal fat mass (AFM).

Download figure to PowerPoint

Reproducibility

The intra- and interobserver reproducibility was tested in 20 different images for the following SCTT parameters: MAFM, MTFM, MALM, MTLM, AFM and SSFM. Two operators (G.L. and E.P.), blinded to each other's and their own recordings, performed three measurements for each SCTT parameter. Precision was assessed as the coefficient of variation of each SCTT parameter (Table 1).

Table 1. Variability of repeated ultrasound measurements of subcutaneous tissue thickness parameters
ParameterCoefficient of variation (%)
IntraobserverInterobserver
  1. AFM, abdominal fat mass; MAFM, mid-arm fat mass; MALM, mid-arm lean mass; MTFM, mid-thigh fat mass; MTLM, mid-thigh lean mass; SSFM, subscapular fat mass.

MAFM8.410.2
MALM7.59.7
MTFM9.29.8
MTLM5.77.0
SSFM7.910.5
AFM8.210.9

Statistical analysis

For every SCTT parameter time-specific reference ranges were computed, according to the procedure described by Royston21. Logarithmic transformations of the SCTT parameters were used to fit quadratic polynomial time-based curves. This was performed for all the parameters investigated with the exception of SSFM and MALM, for which the logarithmic parameter fit a linear time-based curve. We checked ‘normality of model's residuals’ and ‘homoscedasticity’ by means of normal plots and the associated test recommended by Royston21.

To compare the healthy group with the GD group, we plotted the residuals for the GD pregnant women against gestational age (residuals computed using the estimated parameters previously obtained for the healthy group) and we checked for Gaussian distribution of residuals using normal plots and the corresponding statistical test proposed by Royston21. Then, since no trend with gestational age seemed to be present, we performed a classic t-test to compare mean differences of residuals between the two groups at each gestational age.

The study was approved by the institutional review boards at the Tor Vergata University of Rome and at the University of Florence and the local ethics committee of the Fatebenefratelli Hospital-Isola Tiberina of Rome approved the study protocol.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Characteristics of the study groups are summarized in Table 2. Pre-gestational body mass index and birth weight were greater among the group of GD patients.

Table 2. Characteristics of the studied populations
CharacteristicHealthy groupGD groupP*
  • *

    Using t-test. NS, not significant; GD, gestational diabetes.

n21885 
Age (years, mean ± SD)27.4 ± 6.728.2 ± 4.5NS
Prepregnancy body mass index (kg/m2, mean ± SD)24.3 ± 2.925.4 ± 3.20.01
Gestational age at delivery (weeks, mean ± SD)39.0 ± 3.038.6 ± 2.6NS
Birth weight (g, mean ± SD)3283 ± 3953481 ± 416< 0.01

Model-based reference ranges were generated for each fetal parameter obtained in the cross-sectional group of normal patients. Table 3 shows the 5th, 50th and 95th centiles of these distributions. Each parameter increased progressively with advancing gestational age, as expected in a group of normal fetuses in normal pregnant women.

Table 3. Reference ranges of subcutaneous tissue thickness parameters in the healthy pregnant group (n = 218)
ParameterPercentileGestational age (weeks)
20–2223–2526–2829–3132–3435–3738–40
  1. AFM, abdominal fat mass; MAFM, mid-arm fat mass; MALM, mid-arm lean mass; MTFM, mid-thigh fat mass; MTLM, mid-thigh lean mass; SSFM, subscapular fat mass.

MAFM (cm2) 5th0.660.991.431.992.653.404.19
50th1.001.502.163.004.005.136.32
95th1.502.263.264.536.047.749.54
MALM (cm2) 5th0.800.981.201.461.782.182.66
50th1.231.501.842.242.743.344.08
95th1.892.312.823.444.205.136.26
MTFM (cm2) 5th0.881.532.453.645.026.407.56
50th1.332.303.705.507.579.6611.41
95th2.003.485.588.3011.4314.5717.22
MTLM (cm2) 5th1.271.912.733.674.665.596.34
50th1.822.753.915.276.698.039.11
95th2.613.945.627.569.6111.5413.08
SSFM (mm) 5th1.391.611.862.152.492.893.34
50th2.202.552.953.423.954.585.30
95th3.494.044.685.416.277.258.40
AFM (mm) 5th1.391.862.382.903.383.764.00
50th2.092.803.584.385.105.686.03
95th3.154.235.416.607.708.579.10

Figure 2 shows time-specific estimated 90% reference ranges for SCTT parameters for both the normal and the GD groups. Because of the logarithmic transformation, the range is asymmetrical about the regression line and its width increases somewhat with gestational age.

thumbnail image

Figure 2. Gestation-specific reference ranges for the studied subcutaneous tissue thickness parameters within the healthy (solid lines) and the gestational diabetic (dotted lines) groups (groups identified on the basis of oral glucose tolerance test). The 5th, 50th and 95th centiles are shown in each case. The comparison between groups was performed with a classic t-test between residuals, per each gestational age, computed using only the normal reference ranges.

Download figure to PowerPoint

Significant differences were found between the two study groups at 37–40 weeks' gestation for MTFM (11.86 ± 2.70 vs. 14.22 ± 3.52 cm2, P = 0.02), at 20–22 and 26–28 weeks for MTLM (1.82 ± 0.42 vs. 3.00 ± 0.08 cm2, P = 0.04 and 3.91 ± 0.74 vs. 4.29 ± 0.95 cm2, P = 0.05, respectively), at 31–34 and 35–37 weeks for MAFM (4.13 ± 0.89 vs. 4.92 ± 1.13 cm2, P = 0.03 and 5.13 ± 1.57 vs. 6.62 ± 1.70 cm2, P < 0.01, respectively), at 26–28 and 38–40 weeks for SSFM (2.95 ± 0.52 vs. 3.24 ± 1.28 mm, P = 0.03 and 5.30 ± 1.41 vs. 6.73 ± 1.24 mm, P < 0.01, respectively), and at 39–40 weeks for AFM (6.18 ± 1.32 vs. 6.80 ± 0.89 mm, P = 0.03), the means of the GD residuals always being greater than those obtained from the healthy group of pregnant women.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The first aim of the present study was to provide reference values for new parameters of fetal body composition as several studies support the role of fetal fat and lean mass assessment in the determination of normal fetal development5, 6, 16.

Bernstein and Catalano examined the utility of ultrasound to measure subcutaneous fetal fat in the extremities. The use of a simple linear measurement of fat thickness across the extremity was found to be poorly reproducible, with an intraobserver coefficient of variation of 28%18. This appeared to be the result of a distortion in the proximal extremities resulting from external compression. The measurement of fat area in the proximal extremities has proved to be more reproducible. The coefficients of variation for the sonographic estimates of subcutaneous fat area in the present study compare reasonably with published coefficients of variation for the measurement of skinfold thickness in neonates22.

Bernstein et al.5 also reported that fetal fat and lean body mass have peculiar growth profiles and that, as a result of an accelerated rate of growth in late gestation, the measurement of fetal fat will provide a more sensitive and specific marker of abnormal fetal growth when compared with index values of lean body mass. An index of fat as a predictor of morbidity has been widely applied in neonates. In the growth-restricted neonate a low ponderal index (Rohrer's index of corpulence) has a stronger association with several specific morbidities than does birth weight23.

Galan et al.6 recently showed that sonography can be used to follow subcutaneous measurements longitudinally and to detect differences, and potentially disease processes, in study populations. For these reasons we set out to determine SCTT parameters in a population of pregnant women with GD.

Bernstein and Catalano24 reported that increased neonatal fat is associated with an increased risk of Cesarean delivery in infants born to mothers with GD. Moreover subcutaneous fat appears to be a stronger index of maternal glucose control than does the ambulatory glycemic profile24, 25. Perinatal mortality and morbidity are increased among macrosomic fetuses from GD mothers compared with macrosomic fetuses from normal pregnant women26. Therefore the development of an index of fetal fat distribution could be clinically helpful.

Furthermore, in GD the relative risk of shoulder dystocia for a 4000-g fetus appears to be three or four times higher than that observed in a normal population26. Conventional ultrasound fetal biometry has limited value in these clinical situations: when the fetal weight is > 90th centile the measurement error may even reach 15%27.

The second aim of our study was to determine fetal SCTT values for a GD population under standard treatment. The small study size, strict maternal metabolic control, and absence of severe macrosomia in the population are the main factors that must be taken into account when evaluating the reproducibility of these results. Although the normal reference values were obtained from a healthy normoglycemic population, few differences between the two study groups were found. This may be due to the strict control of the diabetic patients (diet, diet plus insulin, frequent blood glucose profiles). Moreover, we assume that GD fetuses receiving an excessive glucose load might show stable features of fat distribution that could be difficult to differentiate from normal fetuses. It is therefore intriguing that an increased MTLM in the GD fetuses was found before the definitive diagnosis of GD. This finding seems to suggest that, although the maternal metabolic maladaptation did not definitively occur at 20–22 weeks, the fetus was probably already affected, showing signs of an excessive availability of nutrients (higher MTLM). Yet the abnormal metabolic influence on the fetus was not evident through the traditional sonographic measurements.

It could be important to evaluate the potential impact of the SCTT on the correct estimation of birth weight. Several birth-weight prediction formulae15 have been provided over the last 15 years; the ability of these formulae to predict fetal weight has always been associated with an error of at least 8–10%. The incorporation of SCTT measurements into existing formulae involving long bone (humerus and femur) and head and abdominal circumference measurements could reduce the amplitude of this error. In cases in which excessive or reduced growth affects primarily soft tissue deposits and not the conventional parameters used to assess fetal growth, the evaluation of SCTT could have a prominent role in the identification of mild nutritional fetal abnormalities.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We thank Dr. Patrizio Pasquletti (Center of Biostatistcs, A.Fa.R., Fatebenefratelli Hospital Isola Tiberina, Rome) and Dr. Beniamino Casalino (Tor Vergata University) for their kind technical assistance.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References