- Objective: The role of steroid hormones in the control of human parturition has been a subject of debate. Activation of the fetal hypothalamic-pituitary-adrenal axis leading to an increase in plasma cortisol is followed by the onset of parturition in sheep. In contrast, androgens, specifically, dehydroepiandrosterone sulfate, have been implicated in the control of parturition in nonhuman primates. The purpose of this study was to determine the relationship between human fetal plasma cortisol and dehydroepiandrosterone sulfate and the onset of preterm labor in patients with preterm premature rupture of the membranes. Study Design: Fetal blood sampling was performed in 51 patients with preterm premature rupture of membranes who were not in labor on admission. Amniotic fluid was cultured for aerobic and anaerobic bacteria and mycoplasmas. Corticosteroids had not been administered before fetal blood sampling. Cortisol and dehydroepiandrosterone sulfate were measured with sensitive and specific immunoassays. Analysis was conducted with nonparametric statistics and survival analysis. Results: (1) Patients who went into spontaneous labor and delivered within 7 days of cordocentesis had a significantly higher median level of fetal plasma cortisol but not of dehydroepiandrosterone sulfate than those delivered after 7 days (for fetal plasma cortisol: median 8.35 [4.7 to 12.4] μg/dL vs median 4.75 [3.0 to 10.4] μg/dL, P < .0001; for fetal plasma dehydroepiandrosterone sulfate: median 154.4 [8.6 to 333.8] μg/dL vs median 194.6 [96.7 to 402.5] μg/dL, P = .09). (2) The cordocentesis-to-delivery interval was significantly shorter in patients with a fetal plasma cortisol value of ≥7 μg/dL (derived by receiver-operating characteristic curve analysis) than in those with fetal cortisol <7 μg/dL (median 49 [4 to 1849] hours vs median 325 [11 to 2590] hours, P < .001). (3) Fetal plasma cortisol, but not maternal cortisol, was an independent predictor of the duration of pregnancy after we adjusted for gestational age and the results of amniotic fluid culture (hazards ratio 2.9, P < .05). (4) There was a significant correlation between fetal plasma cortisol and fetal plasma interleukin-6 (r = 0.3, P < .05). (5) A strong relationship was found between the fetal plasma cortisol/dehydroepiandrosterone sulfate ratio and the interval to delivery (P < .005). Conclusion: An elevation in fetal plasma cortisol but not dehydroepiandrosterone sulfate was followed by the onset of spontaneous preterm labor in patients with preterm premature rupture of the membranes. (Am J Obstet Gynecol 1998;179:1107-14.).