Background. The aim of this prospective, randomized study was to investigate the changes in urinary nitric oxide (NO) metabolite between the latent and the active phases of spontaneous and either prostaglandin E 1 (PGE1) or prostaglandin E2 (PGE 2)-induced labors. Methods. Eighty-eight singleton pregnant women at 36-41+ weeks' gestation without signs of fetal distress were enrolled. The first group consisted of 29 pregnant women in whom labor was induced by PGE1 applied intravaginally. The second group consisted of 29 pregnant women with labor induced by PGE2 applied intracervically. The third group consisted of 30 women, who had spontaneous active labor. Clinical data of the three groups were assessed as labor progressed. Results. Urinary nitric oxide/creatinine (U NO/Cr) decreased significantly after the onset of active labor in all three groups (p <0.005), with the percentage decline of 42.2%, 28.6% and 10.1%, respectively. The magnitude of the difference in decline in U NO/Cr after active labor between the PGE1-induced and the spontaneous labor group was significantly reduced (p = 0.0047) after adjustment for potential confounders using the generalized estimating equations test (GEE). The duration of the latent phase was significantly shortened in the PGE1-induced group as compared with the spontaneous labor group (p <0.01). Conclusions. Decreased U NO/Cr may facilitate transition from the latent to the active phase either in spontaneous or induced labors. Our results indicate that U NO/Cr can serve as an easily obtained marker for use in controlling myometrial contractility and cervical ripening at the onset of active labor. The nitric oxide system is present in the human uterus and may contribute to uterine quiescence during pregnancy and show down-regulation in U NO/Cr at the initiation of active labor.
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