HbA_1c and birthweight in women with pre-conception type 1 and type 2 diabetes: a population-based cohort study

To investigate clinical and sociodemographic predictors of birthweight in singletons born to women with type 1 or type 2 diabetes.

Potentially modifiable independent predictors of increase in birthweight were pre-pregnancy care (adjusted regression coefficient [b] = 87.1 g; 95% CI 12.9, 161.3), increasing third-trimester HbA_1c ≤7% (53 mmol/mol) (b = 310.5 g per 1% [11 mmol/mol]; 95% CI 246.3, 374.7) and increasing maternal BMI (b = 9.5 g per 1 kg/m²; 95% CI 3.5, 15.5). Smoking during pregnancy (b = −145.1 g; 95% CI −231.4, −58.8), later gestation at first antenatal visit (b = −15.0 g; 95% CI −26.9, −3.0) and higher peri-conception HbA_1c (b = −48.2 g; 95% CI −68.8, −27.6) were independently associated with birthweight reduction. Pre-pregnancy nephropathy (b = −282.7 g; 95% CI −461.8, −103.6) and retinopathy (b = −175.5 g; 95% CI −269.9, −81.0) were independent non-modifiable predictors of reduced birthweight, while greater maternal height was a non-modifiable predictor of increasing birthweight (b = 17.8 g; 95% CI 12.3, 23.2). Other predictors of birthweight increase were male sex, multiparity and increasing gestational age at delivery. Type or duration of diabetes, socioeconomic status and ethnicity were not associated with continuous birthweight.

Poor glycaemic control before and throughout pregnancy is associated with abnormal fetal growth, with increasing peri-conception HbA_1c predicting weight reduction and increasing third-trimester HbA_1c predicting increased birthweight. Women with microvascular complications of diabetes may require increased surveillance to detect fetal growth restriction.