Main outcome measures: Levels of glycated haemoglobin (HbA1c) and 25(OH)D measured during the third trimester; maternal age, ethnicity, body mass index (BMI) and occupational status; and results of oral glucose tolerance testing (OGTT).
Results: 147 women with a mean gestational age of 35 ± 2 weeks were included, of whom 41% had insufficient or deficient levels of 25(OH)D (≤ 50 nmol/L). Ethnicity, occupational status and season significantly influenced 25(OH)D levels (P < 0.01 for all) but BMI did not. 25(OH)D levels were inversely associated with fasting and 2-hour blood glucose levels during OGTT (Spearman r = − 0.16; P = 0.05 for both) and with log[HbA1c] (Spearman r = − 0.32; P < 0.001). BMI and insulin doses were also associated with HbA1c levels. Multivariable analysis identified 25(OH)D and blood glucose levels during the OGTT as independent predictors of HbA1c levels.
Conclusions: Lower 25(OH)D levels are independently associated with poorer glycaemic control. Future randomised trials are needed to determine whether vitamin D plays a role in glycaemic control in GDM. Regardless, maternal vitamin D insufficiency has adverse effects including neonatal hypocalcaemia and rickets. The 41% prevalence of inadequate 25(OH)D levels in the women in our study is unacceptably high. We propose routine 25(OH)D testing of all pregnant women at screening for GDM or earlier, and treatment of women who are found to be deficient.
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