The IGF1R is a keystone of fetal growth regulation by mediating the effects of IGF-I and IGF-II. We detected 21 IGF1R defects in patients with fetal growth restriction, including 8 heterozygous CNVs and 10 heterozygous, 1 homozygous, and 1 compound-heterozygous SNVs. We validated a scoring system based on fetal and post-natal growth retardation, microcephaly and elevated IGF-I levels to identify IGF1R defect. We found a sensitivity of 92% for patients with at least 3 out of 4 criteria. In vitro, we demonstrated an impaired function of the mutated receptors. This functional test is useful to assess pathogenicity of variants of unknown significance, especially with the expansion of nextgeneration sequencing technologies. (By Dr Eloïse Giabicani, https://jmg.bmj.com/content/early/2019/10/05/jmedgenet-2019-106328 )