Activin A (βA–βA) and activin B (βB–βB) are related dimeric proteins that regulate numerous cellular activities. Activin activity is bioneutralized by follistatin, a specific and high-affinity binding protein. Recently, our group developed specific and sensitive enzyme-linked immunosorbent activin assays that do not detect either activin isoform when bound to follistatin, therefore, the assays are specific for biologically relevant ligands. Activin A is measurable in the serum of pregnant women (cross-sectional sample collection), while activin B is not detected in maternal serum. However, activin B is measurable in amniotic fluid and cord blood sera. The purpose of this study was to measure serum activin A, activin B, and follistatin prospectively in longitudinally collected samples during pregnancy. This study design offered observations of relative changes in serum hormone concentration with each person serving as an internal reference. Serum samples were collected bimonthly from seven pregnant women beginning within the second month of gestation, and up to, but not including, the onset of labor. Six of the seven women had normal labor and delivery. One patient required pitocin (an oxytocin agonist) for induction of labor which led to delivery. Activin A, activin B, total follistatin, free follistatin, human chorionic gonadotropin, estradiol, progesterone, FSH, and LH were measured in maternal serum samples using specific assays. Serum activin A levels increased in the final month of pregnancy in the six patients who delivered following normal labor (Journal of Endocrinology (1997) 152, 167–174