Labor induction and meconium: in vitro effects of oxytocin, dinoprostone and misoprostol on rat ileum relative to myometrium.

Background: Oxytocin and various prostaglandin preparations are commonly used for pharmacological induction of labor at term. Some prostaglandin preparations appear to be associated with more fetal meconium passage and maternal gastrointestinal side effects than is oxytocin. These adverse effects may be caused by stimulation of bowel smooth muscle in the mother and in the fetus.

Aim: To determine whether the in vitro ileal contractile activities of dinoprostone, misoprostol and oxytocin are in keeping with the variation in rates of meconium staining observed during labor induction with these substances.

Methods: The contractile activity of the drugs was tested on isolated rat uterus and ileum mounted in Tyrode's solution.

Results: Uterine contractions were stimulated by all three drugs, whereas ileal contractions were only stimulated by dinoprostone and misoprostol. Oxytocin had no contractile activity on the ileum. Dinoprostone and misoprostol stimulated significantly (p < 0.05) larger uterine contractions than ileal contractions.

Conclusion: These results provide a pharmacological basis for the increased rate of maternal gastrointestinal complaints and meconium passage with labor induction using dinoprostone and misoprostol compared to oxytocin. Based on the above findings, we postulate that the difference in the rates of meconium passage following misoprostol and dinoprostone administration is caused by pharmacokinetic rather than pharmacodynamic differences between the two drugs.