Many of the human myometrial proteins associated with uterine quiescence and the switch to coordinated contractions at the onset of labor exist as alternatively spliced isoforms. There is now extensive evidence to indicate that the nuclear concentrations of the trans-acting splicing regulators SF2/ASF and hnRNP A1/A1B are fundamental in regulating the expression of specific protein isoforms derived from alternative splicing of single precursor messenger ribonucleic acid transcripts. The question thus arose as to whether these factors were also involved in regulating the expression of specific myometrial protein species within different uterine regions during human gestation and parturition. SF2/ASF and hnRNP A1/A1B expression was therefore determined in paired upper (corpus) and lower segment myometrial samples taken from individual women at term/during spontaneous labor and compared with nonpregnant control samples using specific monoclonal antibodies. We report that SF2/ASF levels were substantially increased in the lower uterine region, and this was associated with a parallel decrease in levels of hnRNP A1/A1B during gestation. Conversely, the opposite pattern was observed within the upper uterine region during pregnancy, where hnRNP A1/A1B was significantly up-regulated and SF2/ASF levels were much less than those found in the lower uterine segment. The differential expression of hnRNP A1/A1B and SF2/ASF in the upper and lower uterine segments may have a primary role in defining the formation of specific myometrial protein species associated with the known contractile and relaxatory properties of these regions before and during parturition.