The following is a summary of “Stillbirth risk and smallness for gestational age according to Hadlock, INTERGROWTH-21st, WHO, and GROW fetal weight standards: analysis by maternal ethnicity and body mass index,” published in the NOVEMBER 2023 issue of Obstetrics and Gynecology by Gardosi, et al.

For a study, researchers sought to assess and compare the efficacy of four globally recognized fetal weight standards in identifying the risk of stillbirth across diverse ethnicities and maternal size categories within a multifaceted population.

The study delved into an extensive dataset, encompassing over 2.2 million pregnancies. Three widely recognized population-based fetal weight standards—Hadlock, Intergrowth-21st, and the World Health Organization standards—were juxtaposed against the customized GROW standard. This GROW standard was tailored by factoring in variables such as maternal height, weight, parity, and ethnic background. The study further scrutinized the incidence of small-for-gestational-age birthweights and the associated risk of stillbirths within two predominant ethnic cohorts: British European, and South Asian. The analysis also segmented these findings across five body mass index categories and four maternal size groupings characterized by a standard body mass index range of 18.5–25.0 kg/m². The comparative trends in stillbirth rates versus small-for-gestational-age rates were quantified using the Clogg z test. Concurrently, differences pertaining to stillbirths across various body mass index categories were evaluated employing the chi-square trend test.

In the study’s findings, it was evident that the stillbirth rates were significantly higher among South Asian pregnancies, registering at 5.51 per 1000, compared to their British-European counterparts, which stood at 3.89 per 1000, showing a clear statistical difference (P<.01). Furthermore, as the body mass index escalated within both ethnic groups, a corresponding increase in stillbirth rates was observed, highlighting a notable trend (P<.01). When assessing the classification of babies as small-for-gestational-age, there was a stark contrast between the rates for South Asian babies and British European babies as per the population-average standards. Specifically, the Hadlock standard showed rates of 26.2% versus 12.2%, the Intergrowth-21st at 12.1% versus 4.9%, and the World Health Organization at 32.2% versus 16.0%. However, the customized GROW standard presented more comparable rates of 14.0% versus 13.6%. Intriguingly, irrespective of the standard applied, pregnancies identified as small-for-gestational-age consistently exhibited a heightened risk of stillbirth when compared to their non-small-for-gestational-age counterparts. Delving into the nuances of different body mass index subgroups, a divergence in trends emerged between the GROW standard and the other population-average standards. Specifically, as stillbirth rates climbed, the GROW standard reflected a rise in the rate of small-for-gestational-age. Conversely, standards like Hadlock, Intergrowth-21st, and the World Health Organization depicted declining rates as body mass index ascended, indicating a distinct trend divergence (P<.01) concerning stillbirth rates across varying body mass index categories, notably, within the subgroup characterized by a normal body mass index, consistent stillbirth rates were observed across diverse maternal size groups. This trend was mirrored in the GROW-based small-for-gestational-age rates. However, the rates derived from population-average standards declined as maternal size increased. 

In conclusion, while evaluating fetal growth charts, it becomes imperative to critically assess their efficacy across varied subgroups of a heterogeneous population. The study underscored that the association between elevated stillbirth risk and higher maternal body mass index is more accurately captured by the customized growth standard than its population-average counterparts.

Source: ajog.org/article/S0002-9378(23)00364-2/fulltext