['The study was conducted to determine whether relaxin has a proliferative effect on amniotic epithelial cells and to show that this effect is caused by its stimulation of the insulin-like growth factor-II (IGF-II) gene.', 'Immunolocalization and Northern analysis were used to confirm the expression of IGF-II by the fetal cells in the membranes. Human amniotic epithelial (WISH) cells were treated with doses of IGF-II or human relaxin and their proliferative effects measured. The mechanism of the effect of relaxin on cellular proliferation was studied with the use of an IGF-II-blocking antibody and Northern analysis for IGF-II gene expression after treatment with relaxin. An in vivo correlate was sought by quantitation of relaxin gene expression in 10 fetal membranes from women with normally grown and large for gestational age infants.', 'The amniotic epithelial and cytotrophoblast cells of the fetal membranes expressed IGF-II, as did the amniotic epithelial-like (WISH) cell line. Treatment of WISH cells with IGF-II or relaxin caused a significant (P <.03) and dose-related increase in WISH cell proliferation over 5 days. The concurrent treatment with a blocking antibody to IGF-II significantly decreased the proliferative response to IGF-II (P <.002) and relaxin (P <.002). Treatment with relaxin caused a significant increase (P <.003) in the transcription of IGF-II in 24 hours. In fetal membranes, the levels of relaxin gene expression correlated with fetal membrane surface area (r = 0.76) and was significantly greater (P <.008) in the membranes from macrosomic infants (4020-4729 g) compared with those normally grown (2855-3830 g).'] Does relaxin cause proliferation of human amniotic epithelium by stimulation of insulin-like growth factor-II?