CSH2 (chorionic somatomammotropin hormone 2) is a placental hormone exclusively expressed during pregnancy 1. It stimulates lactation, promotes fetal growth, and regulates metabolism through activation of the prolactin receptor (PRLR) via zinc-induced dimerization, distinguishing it from growth hormone which utilizes the GH receptor 1. CSH2 belongs to the growth hormone/chorionic somatomammotropin gene cluster at chromosome 17.2, alongside pituitary GH and other placental variants 1. The gene exhibits complex evolutionary patterns shaped by hyperactive gene conversion and population-specific selective pressures, with signatures indicating functional specialization distinct from related cluster members 2. CSH2 undergoes alternative splicing, with isoform 3 particularly associated with cellular phenotypic characteristics in pituitary adenomas 3. While clinical significance remains largely unexplored in normal physiology, CSH2's altered expression patterns have been observed in somatotroph adenomas, where specific isoforms correlate with treatment response to somatostatin analogs 3. The restricted pregnancy-specific expression pattern and unique receptor signaling mechanism underscore CSH2's specialized role in feto-maternal physiological adaptation.