What hormone induces labor?

Hormones That Induce Labor

Oxytocin is the primary hormone that induces labor, both naturally produced by the body and synthetically administered for medical induction. 1

Naturally Occurring Oxytocin

• Oxytocin is produced within the supraoptic nucleus and paraventricular nucleus of the hypothalamus and released from the posterior pituitary lobe into the circulation 1
• During pregnancy, basal levels of oxytocin increase 3-4 fold, preparing the uterus for labor 2
• As labor approaches and progresses, oxytocin is released in pulses with increasing frequency, duration, and amplitude, reaching a maximum of 3 pulses per 10 minutes towards the end of labor 2
• A maximal 3-4 fold rise in oxytocin occurs at birth, followed by additional pulses during the third stage of labor to assist with placental expulsion 2
• The Ferguson reflex is activated when the fetus exerts pressure on the cervix, creating a feedforward mechanism that releases more oxytocin 1

Mechanism of Action

• Oxytocin binds to specific myometrial oxytocin receptors to induce uterine contractions 1
• High levels of circulating estrogen at term increase the sensitivity of these receptors 1
• Oxytocin also stimulates prostaglandin synthesis and release in the decidua and chorioamniotic membranes, which contribute to cervical ripening and uterine contractility 1
• Interestingly, oxytocin peaks during labor do not directly correlate in time with individual uterine contractions, suggesting additional mechanisms in the control of contractions 2

Synthetic Oxytocin for Labor Induction

• Synthetic oxytocin (Pitocin) is the medication of choice for medical induction of labor 3
• The American College of Obstetricians and Gynecologists recommends oxytocin for induction of labor when there are medical indications such as Rh problems, maternal diabetes, pre-eclampsia at or near term, or when membranes are prematurely ruptured 3
• Intravenous infusion is the only acceptable method of administration for labor induction 3
• The initial dose should be no more than 1-2 mU/min, gradually increased in increments of no more than 1-2 mU/min until a normal contraction pattern is established 3
• At recommended doses, synthetic oxytocin typically does not cross the placenta or maternal blood-brain barrier 1

Safety Considerations

• Accurate control of infusion rate is essential, requiring an infusion pump and frequent monitoring of contraction strength and fetal heart rate 3
• The oxytocin infusion should be discontinued immediately in the event of uterine hyperactivity or fetal distress 3
• High doses of oxytocin may induce tachystole and uterine overstimulation, with potentially negative consequences for the fetus and mother 1
• Low-dose oxytocin protocols may have the advantage of less hyperstimulation 4
• The risk of uterine rupture with oxytocin is approximately 1.1%, which is lower than with prostaglandins 5

Clinical Applications Beyond Labor Induction

• Oxytocin is also indicated for stimulation or reinforcement of labor in cases of uterine inertia 3
• After delivery, 5-10 IU of synthetic oxytocin is often routinely given as an intravenous or intramuscular bolus to control postpartum bleeding or hemorrhage by inducing uterine contractility 3, 1
• In women with respiratory disease, oxytocin is the uterotonic of choice for active management of the third stage of labor, as alternatives like ergotamine may cause bronchospasm 6

Physiological Benefits

• Oxytocin released into the brain during natural labor (not synthetic administration) has been implicated in decreasing maternal levels of fear, pain, and stress 2
• Natural oxytocin release during labor may be associated with long-term behavioral and physiological adaptations in the mother and infant 1

Oxytocin remains a highly successful and safe agent for inducing labor with a fairly large therapeutic index when used appropriately 7. However, it should always be employed with care and an understanding of its limitations and potential risks 7.