Reduction of serum citrulline levels in women at term toward the day of labor onset

Objective. To test the hypothesis that labor onset could be the result of a reduced release of nitric oxide (NO). Study design. Out of 91 consecutive healthy nulliparous women at term serum citrulline (Cit) and arginine (Arg) levels were measured at least twice in 37 subjects, by the means of HPLC with fluorometric detection. Twenty cases underwent a spontaneous onset of labor (group A) while in 17 cases labor was induced (group B) because of pregnancy prolongation or amniotic fluid reduction. Results. Cit and Arg levels were unaffected by the gestational age. In group A, Cit levels undergo a progressive decrease toward the day of labor (from 32.9 +/- 4.7 mu M/L the day -18,-5 to 27.7 +/- 7.8 the day -4,0; p=0.012) whilst they remained stable in group B (from 33.3 +/- 7.7 to 34.2 +/- 9.2). No significant changes were observed in Arg levels. Cit/Arg ratio remained stable in group A whereas it showed a trend to increase in group B. Conclusion. These data indirectly suggest a reduced release of NO toward term. This phenomenon could play a permissive role in the spontaneous onset of labor of healthy nulliparous women.