Birthweight of babies born to mothers with type 1 diabetes: is it related to blood glucose control in the first trimester?

A retrospective study of 133 pregnancies in women with Type 1 diabetes was performed, and the 116 which progressed beyond 28 weeks were further analysed. Despite good maternal blood glucose control (mean (± SE) HbA1 levels 8.6 ± 0.2 % at the end of the first trimester; 6.9 ± 0.2 % at delivery; normal range 4.0–8.5 %), 38 % of babies had birthweights above the 90th centile and operative intervention occurred in 77 deliveries (66 %). There was no significant correlation between birthweight and HbA1 level at any stage of pregnancy, but mothers with babies above the 90th centile for weight had a higher HbA1 at the end of the first trimester than mothers with babies below the 90th centile (9.3 ± 0.5 vs 7.9 ± 0.2 %, p < 0.05). In contrast there was no difference in the HbA1 levels at delivery (7.0 ± 0.3 vs 6.8 ± 0.2 %). The perinatal mortality rate was 17.7 per 1000 births. The results confirm that in Type 1 diabetes large babies are common despite good blood glucose control, and suggest that maternal blood glucose control in the first trimester may be an important determinant of birthweight.