National trends in offspring birth size in pregnancies with maternal type 1 diabetes in Sweden

Objective: Despite major medical and public health efforts to reduce excess fetal growth in type 1 diabetes (T1D), large population-based studies assessing the population-level effects are lacking. We assessed temporal trends in the overall distribution of birthweights (BW) and ponderal index (PI) in offspring of T1D mothers, and the impact of maternal BMI. Research Design, and methods: Population-based cohort study of all live-born infants to T1D mothers between 1998 and 2016 across Sweden, using data from national registers. Main outcome measures were percentiles of BW and PI. We compared the first 10 years, 1998–2007 (period 1), with the last 10 years, 2008–2016 (period 2), using quantile regression and adjusting for maternal characteristics and hypertensive disease between periods. Results: Among 6,485 offspring of T1D mothers, the birth weights and PI distributions did not change between the two study periods, neither for the heaviest infants, LGA, nor for the smallest infants, independent of maternal first trimester BMI. The proportion of LGA remained at 61%. Conclusion: There was no improvement over a 20-year period in the distributions of birth weights or PÍs in offspring of T1D mothers in Sweden, unrelated to maternal BMI. Overall, 61% of offspring of T1D mothers were LGA.