INTERGROWTH-21st v. local South African growth standards (Theron-Thompson) for identification of small-for-gestational-age fetuses in stillbirths: A closer look at variation across pregnancy.

Background: Global growth standards for fetuses were recently developed (INTERGROWTH-21st). It has been advocated that professional bodies should adopt these global standards.
Objectives: To compare the ability of INTERGROWTH-21st with local standards (Theron-Thompson) to identify small-for-gestational-age (SGA) fetuses in stillbirths in the South African (SA) setting.
Methods: Stillbirths across SA were investigated (>500 g, 28 - 40 weeks) between October 2013 and December 2016 (N=14 776). The study applied the INTERGROWTH-21st standards to classify stillbirths as <10th centile (SGA) compared with Theron-Thompson growth charts, across pregnancy overall and at specific gestational ages.
Results: The prevalence of SGA was estimated at 32.2% and 31.1% by INTERGROWTH-21st and Theron-Thompson, respectively. INTERGROWTH-21st captured 13.8% more stillbirths as SGA in the earlier gestations (28 - 30 weeks, p<0.001), but 4.0% (n=315) fewer between 33 and 38 weeks (p<0.001). Observed agreement and the Kappa coefficient were lower at earlier gestations and at 34 - 36 weeks.
Conclusions: Our findings demonstrated differences in the proportion of stillbirths considered SGA at each gestational age between the INTERGROWTH-21st and the local SA standard, which have not been considered previously by other studies.