Phenotypes of fetal macrosomia and risk of stillbirth among term deliveries over the previous four decades.

Objectives: To evaluate the association between macrosomia and stillbirth over the previous four decades and to determine the consistency of the relationship.

Methods: This was a population-based retrospective cohort study using United States Natality and Fetal Death Data from 1982 to 2017 and restricted to the gestational age range of 37-41 weeks inclusive. Macrosomia was defined as birthweight ≥4000 g and subdivided into its grades as previously recommended: grade 1 (4000-4499 g), grade 2 (4500-4999 g), and grade 3 (≥5000 g). We calculated temporal trends of stillbirth among fetuses with macrosomia over the years using joinpoint regression. We generated odds ratios from adjusted binomial logistic regression models to examine the association between macrosomia and risk of stillbirth stratified by grades using normal-weight infants (2500-3999 g) as referent.

Results: Within the fetal macrosomia group, the rate of stillbirth declined from 2.04/1000 in 1982 to 1.05/1000 by the end of the study period (2017), representing a drop of about 48.5%. For the normal-weight fetuses, stillbirth rate declined from 1.95/1000 to 0.83/1000, equivalent to a decline of 57.4%. Macrosomia was significantly associated with elevated risk for stillbirth: grade 2 (OR = 1.27; 95% CI = 1.22-1.32) and grade 3 (OR = 5.97; 95% CI = 5.69-6.22).

Conclusions: Fetal macrosomia is a significant risk factor for fetal demise with the worst intrauterine survival observed among those classified as grade 3. Fetal macrosomia is a heterogeneous rather than a homogeneous entity in terms of risk profiles, and this needs to be considered in future policy guidelines.