Oxytocin and the initiation of parturition. A review.

Oxytocin binds to receptors in myometrial cells, triggering contractions with minimal increases in maternal oxytocin levels.

• Myometrial cells have increased sensitivity to oxytocin, especially near term due to higher receptor concentrations.
• Factors influencing the level of oxytocin receptors may include steroid levels and the presence of other hormone receptors.
• Maternal plasma oxytocin levels remain relatively stable throughout pregnancy.
• The fetus secretes significant amounts of oxytocin during the first stage of labor, which can affect the myometrium despite placental oxytocinase activity.
• At the second stage of labor, distension of the lower birth canal may stimulate oxytocin release from the maternal neurohypophysis, potentially enhancing contractions.

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