Objective: To evaluate whether levels of placental growth hormone (GH) and Insulin-like Growth Factor-I (IGF-I) are associated with development of LGA infants in pregnant women with type 1 diabetes. Design: Observational study of 103 consecutive pregnant women with long-term type 1 diabetes and median HbA1c 6.6% (range 4.9-10.5) (49mmol/mol (30-91)) in early pregnancy. At 8, 14, 21, 27 and 33weeks weight was recorded and blood was sampled for measurements of placental GH, IGF-I and HbA1c. LGA was defined as birth weight >90th percentile after adjustment for gender and gestational age. Results: Throughout pregnancy placental GH levels were similar in 51 (50%) women delivering LGA infants compared with the remaining women except at 8. weeks where placental GH levels were lower in women with LGA infants (1.1. ng/ml (0.1-4.3) vs. 1.7 (0.3-11.7), p = 0.04). IGF-I levels were similar in women with and without LGA infants (p = 0.97). Gestational age at first blood sampling was similar in women with and without LGA infants (60. days (37-89) vs. 61.5 (42-94), p = 0.42). Placental GH levels at 14. weeks correlated negatively with weight gain in early pregnancy (r = -0.32, p = 0.002). As predictors of LGA infants, multivariate logistic regression analysis identified placental GH levels at 8. weeks (OR 0.4 (95% CI: 0.2-0.9), p = 0.02), HbA1c at 33. weeks (3.6 (1.3-9.9), p = 0.01) and parity ≥. 1 (3.1 (1.3-7.5), p = 0.01) after adjustment for pre-pregnancy BMI. Conclusions: Women delivering LGA infants had lower placental GH levels in early pregnancy. Growth factors and maternal weight gain in early pregnancy may be important for healthy fetal growth.