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Serum plasma activin-A is measurable in the maternal circulation of healthy pregnant women, increases in specimens collected during the third trimester of gestation, and is highest at parturition. Hormone abnormalities are known to be associated with preterm labor or diabetes in pregnancy. Therefore, in the present study serum activin-A levels in normal controls were compared to those in pregnant women with preterm labor or gestational diabetes. In some cases, values were obtained before and after insulin therapy. In other controls and patients with preterm labor, the activin-A concentration in cord serum was also studied. A newly developed two-site immunotest was used to determine activin-A levels. Subjects included normal controls (n = 7), who were sampled throughout gestation every 5 weeks; pregnant women at term (38-40 weeks) not in labor (n = 22); pregnant women at term in spontaneous labor (< 3.0 cm dilated; n = 42); women in preterm labor (25-35 weeks; n = 38); and women with gestational diabetes (20-39 weeks; n = 9). In control women, serum activin-A levels increased from 4.8 +/- 5.5 micrograms/L (mean +/- SD) at 20 weeks to 25.4 +/- 27.8 micrograms/L at 40 weeks (P < 0.01), and values correlated with gestational age. Pregnant women in preterm labor had serum activin-A concentrations (89.04 +/- 173.31 micrograms/L) higher than those in normal controls (P < 0.01), and no significant correlation to gestational age was found in this group of pregnant women. Healthy women in labor showed serum activin-A concentrations higher than those in women at term but not in labor (P < 0.01). Diabetic patients had serum activin-A concentrations (52.39 +/- 23.32 micrograms/L) significantly higher than those in normal controls. In these patients, maternal serum activin-A concentrations significantly decreased to the range in healthy controls at the same gestational age after insulin therapy (9.48 +/- 3.82 micrograms/L). The present study shows that preterm labor is associated with increased concentrations of activin-A in the maternal circulation and cord serum. Hypersecretion of activin-A is also shown in same patients with gestation diabetes; this reverts to normal after insulin treatment.