Background/Aims: To investigate changes in free insulin-like growth factor I (IGF-I) and IGF-binding protein 1 (IGFBP-1) complexed IGF-I during human pregnancy. Methods: Overnight fasting serum was obtained in a longitudinal design from 11 women with non-complicated pregnancy at gestation weeks 6–10, 16–20, 24–28 and 35–38 and, for comparison, 5 weeks post-partum. All samples were analyzed for total and free IGF-I and IGF-II, IGFBP-3 and IGFBP-3 proteolysis, total and non-phosphorylated (np-) IGFBP-1, and IGFBP-1 complexed IGF-I. Results: Total IGF-I was increased in late pregnancy (week 35–38) (p < 0.001), whereas free IGF-I was significantly increased by 77% already at week 6–10 (p = 0.004) and by 140% (p = 0.002) at week 34–38, when compared to post-partum levels. At weeks 16–20 and 24–28, levels of free IGF-I were not significantly different from post-partum levels. Significant IGFBP-3 proteolysis was detectable from week 6–10 and throughout pregnancy (p < 0.05). Total and np-IGFBP-1 were significantly increased from 16–20 weeks of pregnancy (both p < 0.05) and IGFBP-1 complexed IGF-I was increased 2-fold from week 16–20 and throughout pregnancy (p < 0.05). However, the saturation of IGFBP-1 remained constant at 27–29% during the study. Conclusion: We found evidence of increased free IGF-I and increased IGF-I in binary complexes during pregnancy, possibly caused by IGFBP-3 proteolysis and decreased ternary complex formation.

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