Length‐for‐age and weight‐for‐age z scores at birth using the World Health Organization Growth Standards versus the new INTERGROWTH 21st Newborn Size Standards
Authors: Perumal Nandita , Shi Joy , Bassani Diego , Al‐Mahmud Abdullah , Islam M. Munirul , Ahmad Tahmeed , Roth Daniel 
Published: November 2023

The International Fetal and Newborn Growth Consortium for the 21st Century recently published gestational age (GA) specific Newborn Size Standards (IG‐NS). These standards are intended to replace previous newborn size references and complement the World Health Organization Child Growth Standards (WHO‐GS) in clinical and research settings. However, in the context of longitudinal epidemiologic studies or repeated cross‐sectional surveys of postnatal child growth, there are unclear implications of using IG‐NS at birth when the WHO‐GS (which are not GA‐specific) are applied at subsequent postnatal ages. In this study, we aimed to estimate and compare length‐for‐age (LAZ) and weight‐for‐age z scores (WAZ) at birth using IG‐NS versus WHO‐GS among 559 infants born at ≥37 weeks GA (259 – 300 days), enrolled in an ongoing prenatal vitamin D intervention trial in Dhaka, Bangladesh. Prevalence of stunting (LAZ <−2SD) and underweight (WAZ <−2SD) was estimated overall and within GA strata [early‐term (370/7 to 386/7wk), term (390/7 to 406/7 wk), and late‐term (410/7 to 416/7 wk)]. Compared to WHO‐GS, mean (±SD) LAZ using IG‐NS was significantly higher overall (IG‐NS vs WHO‐GS: −0.93 ± 1.07 vs −1.14 ± 1.07; P<0.001) and among early term infants (n=220: −0.84 ± 1.11 vs −1.45 ± 1.13, P<0.001), but lower among late‐term infants (n=32: −1.36 ± 0.74 vs −0.85 ± 0.68, P<0.001). Mean WAZ using IG‐NS (vs WHO‐GS) was similar overall (−1.20 ± 0.86 vs −1.19 ± 0.79; P=0.71) but was significantly higher among early‐term (−0.98 ± 0.83 vs −1.37 ± 0.78; P<0.001) and lower among late‐term infants (−1.65 ± 0.85 vs −1.04 ± 0.84; P<0.001). The overall prevalence of stunting was significantly lower using IG‐NS (14% vs 19%; P<0.001); the proportion of infants classified as stunted was most similar using IG‐NS vs WHO‐GS in the term‐GA category (n=295: 12% vs 12%; P=1.0). Underweight prevalence was similar using IG‐NS vs WHO‐GS overall (18% vs 16%; P=0.336), but was lower among early‐term infants (10% vs. 21%; P<0.001) and higher among term‐GA infants (20% vs 13%; P<0.001). Compared to IG‐NS, the sensitivity of WHO‐GS to classify infants as stunted and underweight at birth was 84% and 69%, respectively; and varied substantially by GA categories. In conclusion, LAZ and WAZ of infants born at term (37–41 weeks GA) may substantially differ according to IG‐NS and WHO‐GS. As expected, differences were most evident for early‐ and late‐term infants. However, overall population averages were also affected. Further research will evaluate the implications of integrating two distinct growth standards – IG‐NS (birth) and WHO‐GS (postnatal) – in epidemiological studies of child growth.Support or Funding InformationNP was supported by the Canadian Institutes for Health Research Sir Frederick Banting and Charles Best Doctoral Award. Data used for this study were collected for the Maternal vitamin D supplementation during pregnancy and lactation to promote infant growth in Dhaka, Bangladesh (MDIG) trial funded by the Bill and Melinda Gates Foundation.