Both macrosomic and growth-retarded fetuses are at an increased risk of perinatal morbidity and mortality . An accurate estimation of fetal weight helps obstetricians in making decision on the route of delivery . Incorporation of fetal soft tissue parameters improves the accuracy of fetal weight estimation . The aim of this study was to assess the accuracy of fetal abdominal subcutaneous tissue thickness as an indicator of fetal birth weight.
The current study concluded that FASTT showed a high statistically significant correlation with EFW by Hadlock formula and BW (birth weight); also, a high statistically significant difference between each of the birth weight categories regarding the value of EFW by Hadlock formula as well as by FASTT was noted. Both EFW and FASTT showed higher values in LGA category than AGA and SGA categories and also showed higher values in AGA category than SGA category
Bhat et al.  also agreed with our results; they plotted birth weight against FASTT (scatter plot graph), and it showed a positive significant correlation between FASTT and birth weight obtained by Pearson’s correlation coefficient (r = 0.418). Similarly, Grace et al.  demonstrated that FASTT may be useful in the assessment of fetal nutritional risk as they showed a significant correlation between subcutaneous tissue thickness, estimated fetal weight, and actual BW.
Regarding the statistically significant difference of FASTT in different birth weight categories, Odthon et al.  showed similar results; they studied the correlation between FASTT and birth weight. The mean FASTT differed significantly between normal and macrosomic fetuses (6.6 mm versus 12 mm, respectively; p < 0.001).
Singh et al.  agreed to our results; the birth weight was divided according to the percentile in three groups according to which results were statistically significant (p < 0.001). Average subcutaneous tissue thickness in babies having a birth weight between 10th and 90th percentile was 5.4 mm. Below 10th percentile was 4.4 mm, and above 90th percentile was > 5.9 mm.
Additionally, the present study results were in accordance with the results recorded by Bhat et al. , who found that the difference in mean FASTT between SGA and AGA babies was statistically significant (p value 0.032). Similarly, the difference in mean FASTT between LGA and AGA babies was also statistically significant (p value 0.000).
Regarding the demographic data of the included subjects, the current study showed no correlation between FASTT and any of the maternal age, gravidity, and parity; however, a statistically significant correlation was noted between the FASTT and gestational age calculated by date (r = 0.79, P value = 0.00). Results of Chen et al.  and Farah et al.  are in agreement with the current study; both found that FASTT measurements increase as gestation advances.
FASTT demonstrated higher sensitivity in LGA (90.9%) than SGA (86.9%) denoting that FASTT is a better indicator of LGA than SGA. The best cutoff value of FASTT for LGA was 9.2 mm and that of SGA was 4.5 mm. Cutoff points of FASTT for LGA and SGA varied in different studies. Close to this result was Mack et al.  who found that the best cutoff value for FASTT in detecting macrosomia was ≥ 10.0 mm with high sensitivity (81.0%) and specificity (86.8%).
Despite that, Bhat et al.  also showed that FASTT was sensitive to predict large for gestational age (LGA) and not sensitive for SGA; a quite different cutoff value of FASTT for large babies was obtained (6.25 mm). Sensitivity for FASTT > 6.25 mm for large for gestational age babies was 79% and specificity is 70%. They also stated that FASTT measurement for the prediction of small babies with birth weight < 2500 g was not sensitive. Therefore, a cutoff value of FASTT for small for gestational age babies could not be obtained.
Regarding SGA, the results of the current study were comparable to the results obtained by Kongsing et al. , who found that the best cutoff value of the subcutaneous fat thickness for prediction of IUGR was 4.5 mm, giving the sensitivity, specificity, positive predictive value, and negative predictive value of 76.0%, 75.3%, 47.5%, and 91.4%, respectively.
Close to our results was Prasertcharoensuk et al.  who found that fetuses with FASTT ≤ 4 mm were more likely to have low birth weight with a sensitivity of 90.0% (95% CI = 86.8–93.3) and a specificity of 53.5%.
As FASTT showed high statistical correlation with AC among the other fetal biometric parameters, comparing AC versus FASTT in cases of LGA was done; this is in conformity with Odthon et al.  who evaluated the value of the sonographic measurement of fetal AC and FASTT for predicting fetal macrosomia. Compatible results were obtained. Our study showed higher values for AC than FASTT (accuracy of 96% for AC against 92% for FASTT) as well as Odthon et al.  study (accuracy of 93.4% for AC against 86.4% for FASTT) denoting that AC is still a better parameter for detection of LGA.
This study showed no statistical correlation between fetal gender and FASTT denoting that fat deposition in the fetus is undependable from the fetal gender. Similarly, Farah et al.  demonstrated that the FASTT increases at the same rate for both male and female fetuses and at any given week.
In the current study, correlation between FASTT and mode of delivery was done; despite 91.6% of pregnant women who had fetuses with FASTT more than 9.2 mm delivered by CS, no statistical significant difference between FASTT and mode of delivery could be detected.
In accordance with the result of this study, Grace et al.  agreed that no direct relationship between FASTT and mode of delivery could be found. Also, Assimakopoulos et al.  found that fetuses with low FASTT were more likely to be delivered through normal vaginal delivery (7.8 ± 0.1 mm), while higher FASTT was correlated with instrumental vaginal delivery (7.9 ± 0.2 mm) and cesarean section (8.6 ± 0.3 mm) (ANOVA, P = 0.034). With increasing FASTT, the likelihood of instrumental vaginal and cesarean delivery increased but with no direct significant statistical correlation.
As FASTT showed a positive correlation with a wide range of fetal weights, it can be incorporated into ultrasonographic weight estimation formulas to achieve better accuracy; it can be used as an alternative to AC in modified formulas when accurate AC measurement cannot be obtained. The cutoff values may serve as a fast predictor for macrosomia and IUGR for pregnant women in labor. The study was limited by the difficulty of measuring FASTT in obese patients, in cases with anteriorly situated placenta, cases of twins, and when the fetal presentation was occipito-anterior.