To estimate the residual risk associations between hyperglycemia and adverse pregnancy outcomes after glycemia-controlling intervention.

Among 41,067 Chinese women, those with gestational diabetes mellitus (GDM), according to the IADPSG criteria, received standard interventions to control glycemia. Risk associations of plasma glucose (PG) levels with excess newborn birth weight, primary cesarean section, and preterm delivery were estimated and compared with those in the Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) study, where hyperglycemia was left untreated.

A total of 6,709 (16.3%) women developed GDM and thus received predominantly lifestyle interventions. The incidence of excess newborn birth weight, primary cesarean section, and preterm delivery was 6.1%, 19.1%, and 4.0%, respectively. Higher fasting and higher post-load PG levels during 75-g oral glucose tolerance test (OGTT) were statistically significantly associated with increased risks of excess newborn birth weight and pre-term delivery. Compared with the HAPO study, the association of fasting PG level with excess newborn birth weight showed similar strength and dose-response pattern, contrasting with considerably weakened associations for post-load PG levels that involved glycemic control. Contrary risk associations were seen across GDM subtypes - compared with non-GDM, isolated fasting GDM was associated with increased, whereas isolated post-load GDM was associated with decreased, risks of excess newborn birth weight and primary cesarean section. Limiting the analysis to non-GDM women and GDM women with low HbA1c (<6.0%) ≥30 days after interventions overall attenuated the risk associations.

Residual risk associations exist between hyperglycemia and adverse pregnancy outcomes despite seemingly appropriate glycemic control.