Outcome of pregnancy oral glucose tolerance test and preterm birth.

BACKGROUND:Gestational diabetes is associated with adverse outcomes such as preterm birth (<37 weeks). However, there is no international consensus on screening criteria or diagnostic levels for gestational diabetes, and it is unknown whether body mass index (BMI) or obesity modifies the relation between glucose level and preterm birth. METHODS:We studied a pregnancy cohort restricted to two Danish regions from the linked Danish Medical Birth Register to study associations between glucose measurements from the 2-hour post-load 75-gram oral glucose tolerance test (one-step approach) and preterm birth from 2004-2018. In Denmark, gestational diabetes screening is a targeted strategy for mothers with identified risk factors. We used Poisson regression to estimate rate ratios (RR) of preterm birth with z-standardized glucose measurements. We assessed effect measure modification by stratifying analyses and testing for heterogeneity. RESULTS:Among 11,337 pregnancies (6.2% delivered preterm), we observed an adjusted preterm birth RR of 1.2 (95% CI: 1.1-1.3) for a 1 standard deviation glucose increase of 1.4 mmol/L from the mean 6.7 mmol/L. There was evidence for effect measure modification by obesity, e.g., adjusted RR for non-obese (BMI <30): 1.2 (95%CI: 1.1-1.3) vs. obese (BMI ≥30): 1.3 (95%CI: 1.2-1.5), P=0.05 for heterogeneity. CONCLUSIONS:Among mothers screened for gestational diabetes, increased glucose levels, even those below the diagnostic level for gestational diabetes in Denmark, were associated with increased preterm birth risk. Obesity (BMI ≥30) may be an effect measure modifier, not just a confounder, of the relation between blood glucose and preterm birth risk.