Abnormal Maternal Body Mass Index and Customized Fetal Weight Charts: Improving the Identification of Small for Gestational Age Fetuses and Newborns

Abstract

Background: Obesity and thinness are serious diseases, but cases with abnormal maternal weight have not been excluded from the calculations in the construction of customized fetal growth curves (CCs).

Method: To determine if the new CCs, built excluding mothers with an abnormal weight, are better than standard CCs at identifying SGA. A total of 16,122 neonates were identified as SGA, LGA, or AGA, using the two models. Logistic regression and analysis of covariance were used to calculate the OR and CI for adverse outcomes by group. Gestational age was considered as a covariable.

Results: The SGA rates by the new CCs and by the standard CCs were 11.8% and 9.7%, respectively. The SGA rate only by the new CCs was 18% and the SGA rate only by the standard CCs was 0.01%. Compared to AGA by both models, SGA by the new CCs had increased rates of cesarean section, (OR 1.53 (95% CI 1.19, 1.96)), prematurity (OR 2.84 (95% CI 2.09, 3.85)), NICU admission (OR 5.41 (95% CI 3.47, 8.43), and adverse outcomes (OR 1.76 (95% CI 1.06, 2.60). The strength of these associations decreased with gestational age.

Conclusion: The use of the new CCs allowed for a more accurate identification of SGA at risk of adverse perinatal outcomes as compared to the standard CCs.

Keywords: birthweight; customized growth charts; fetal weight; maternal body mass index; newborn weight; obesity; perinatal outcomes; small for gestational age; thinness.

Figure 1

Classification of newborns by birth weight according to the use of conventional customized curves (CCs) and new CCs (new CCs). SGA_CC or SGA_{NEW CC}, small for gestational age according to the use of conventional curves (CCs) or new, customized curves (new CCs), respectively). ${\mathrm{SGA}}_{CC}^{only by}$: Infants classified as SGA according to the CCs and as AGA according to the new CCs. ${\mathrm{SGA}}_{New C{C}_{}}^{only by}$: Infants classified as SGA according to the new CCs and AGA according to the CCs. OR₁: Odds ratio of ${\mathrm{SGA}}_{CC}^{only by}$, ${\mathrm{SGA}}_{{}_{NEW} CC}^{only by}$, and ${\mathrm{SGA}}^{by both CC}$ versus ${\mathrm{AGA}}^{by both CC}$ for the composite outcome 1 (cesarean section, Apgar < 7 at 1st min, Apgar < 7 at 5th min, NICU admission or mortality). OR₂: Odds ratio of ${\mathrm{SGA}}_{CC}^{only by}$, ${\mathrm{SGA}}_{New C{C}_{}}^{only by}$, and ${\mathrm{SGA}}^{by both CC}$ versus ${\mathrm{AGA}}^{by both CC}$ for composite outcome 2 (Apgar < 7 at 1st min, Apgar < 7 at 5th min, NICU admission or mortality).* p value < 0.05 and *** p value < 0.001.