Fetal weight estimation by ultrasound is important to the extent that truly reflects the fetal nutritional status. A fetus incorrectly classified as SGA or LGA will induce the clinician to intensify the monitoring of the pregnant woman and, in the specific case of the GDM, even to modify the diet or insulinize the pregnant woman. Therefore, we believe that the clinician should choose the curve that best identifies newborns with true alterations in nutritional status (malnutrition or overnutrition).
This study shows that in newborns of mothers with GDM, the rates of SGA and LGA differ by the reference curve used, INTERGROWTH21st or customized. The SGA rate using INTERGROWTH21st was 4.7%, significantly lower than 10.7% observed using customized curves. In contrast, the LGA rate using INTERGROWTH21st was 25.6%, compared to 13.2% using our customized curves as the reference. These SGA results were consistent with those recently published by Francis et al. [15] who reported overall SGA and LGA rates of 10.5% and 9.5%, respectively, using customized curves. However, our higher LGA rate (13.2% versus 9.5%) may due to the difference in the sample characteristic; Francis et al. included an unselected study population of pregnant women, whereas our study included only pregnant women with GDM, and they are more likely to birth LGA newborns. Using INTERGROWTH21st Francis et al. observed an overall SGA rate, 4.4%, very similar to the 4.7% rate of our sample. However, using INTERGROWTH21st, Francis et al observed a lower LGA rate, 20.6%, than the 25.6% found by us, perhaps due to our inclusion of exclusively mothers with GDM, whereas Francis et al. evaluated an unselected sample.
Similarly, Anderson et al. [25], reported a significantly lower SGA rates using INTERGROWTH21st versus customized curves (4.5% vs 11.6%); LGA rates were not assessed in this general obstetric population.
In addition, we found that the SGA and LGA classifications by each method (customized vs INTERGROWTH21st) reflect differences in their ability to identify true alterations in the neonatal nutritional status, as indicated by the ponderal index. We found that in newborns of mothers with GDM, the RR, 4.24, of malnutrition (PI <10th percentile) in newborns classified as SGA by customized curves was higher, than that of newborns classified as SGA by INTERGROWTH21st, RR = 2.5.
Likewise, the accuracy of the customized curves for identification of malnutrition was greater than that of INTERGROWTH21st, LR + of 3.86 vs 2.74, respectively. That is, using customized curves, it is 3.86 times more likely that a malnourished newborn is classified as SGA than a normally nourished newborn is classified as SGA. The customized curves were also more accurate than INTERGROWTH21st for identifying severe malnutrition, LR + of 5.36 versus 3.86, respectively.
In a previous study by our team [17], carried out in an unselected population, the customized method was superior to the population-based for the identification of newborns with malnutrition. This superiority of the customized method was more evident in the highest scales of maternal weight and height.
Owen et al. [26] found a similar relationship between customized birth weight percentiles and neonatal malnutrition, but concluded that, in a low-risk population, the customized curves are only moderately useful in the identification of neonates with a low PI, with a positive likelihood ratio of 4.3 (95% CI: 2.5–7.1). Agarwal et al [27] also found that the PI at birth was lower in newborns classified as SGA by customized curves than in SGA according to population curves.
Similarly, the RR of overnutrition (PI> 90th centile) associated with LGA classification by customized curves, RR 5.26, was greater than in the newborns classified as LGA by INTERGROWTH21st, RR 3.57). Further, our analysis of the accuracy of each method for identification of overnutrition revealed that the customized method had a greater LR+, 5.40, than the LR+, 2.54, using INTERGROWTH21st. Hence, using customized curves, it is 5.40 times more likely that an over nourished newborn will be classified as LGA than a normally nourished newborn will be classified as LGA. For identification of severe overnutrition, we observed an even greater between method difference: the LR+ of the customized method was much greater, 8.10, than that of INTERGROWTH21st, 3.74. This indicates that using customized curves, it is 8.10 times more likely that a severely over nourished newborn is classified as LGA than a normally nourished newborn is classified as LGA. Given that in GDM it is critical to identify fetal overnutrition, we consider of special relevance the differences found in the PPV of both methods to identify overnutrition. Using our customized curves, the probability that a fetus classified as LGA suffers from overnutrition is 51.61% while using INTERGROWTH21st the probability drops to 32.20%. In the same way, using our customized curves, a fetus classified as LGA is twice as likely to be severely over nourished as if we were using INTERGROWTH21st (28.57% vs. 14.89%). Estos resultados son coherentes con los hallados por Gonzalez et al [28]
The relatively small sample lead to our primary limitations, including occasional RRs with overlapping or wide confidence intervals, which hampered their interpretation. However, the relative risks were usually large enough to be taken clinically relevant. In addition, selection and information biases could affect the estimated of the performance of the two reference curves. We believe that our results can be extrapolated to other populations of pregnant women with adequate monitoring because obstetricians, endocrinologists, family doctors and primary care midwives monitored the pregnant woman with GDM using criteria for diagnosis, follow-up and treatment established by the Spanish Society of Gynecology and Obstetrics.