Induction and Augmentation of Labor
Labor induction is one of the most common obstetrical interventions, occurring in approximately 20% to 30% of all deliveries in developed countries; this also includes elective labor induction in which women choose to be induced but may not have a specific medical reason. Applying this range to United States (U.S.) birth data, this would indicate that approximately 760,000 to 1.1 million labors were induced in the U.S. in 2018.
Labor is unpredictable; just like a cesarean delivery, inductions can be both planned and unplanned:
Some women may know for most of their pregnancy they will likely be induced for either an underlying health condition or certain risk factors that could make spontaneously labor too risky.
Other women may find out during one of their final appointments they need to be induced due to an unexpected health concern.
Other women may end up choosing an induction at 40 or 41 weeks of pregnancy when they previously had no plan to do so.
In any scenario, women should understand how the process works, what options are available, and the risks and benefits to each method. Then women, together with their HCP, can have an in-depth discussion about what will work best for their pregnancy.
Induction and its current processes have been shown to be safe and effective by more than 100 randomized trials (gold standard in research) in viable term inductions.
An induction– usually at 39 weeks or later – can be recommended for numerous reasons:
Postterm pregnancy (most common)
Non-reassuring fetal health status
Membranes have ruptured but labor has not begun
An ultrasound reveals amniotic fluid volume is too low
A woman lives far from her hospital/birthing center
The woman has a history of very quick deliveries
Women may also specifically choose an induction – known as an elective induction, or induction on maternal request.
Induction is usually not recommended in women with a prior cesarean section, placenta previa, the baby is breech, there is an active infection that could transfer to the fetus through vaginal delivery, or umbilical cord prolapse is present. However, in women with a prior cesarean, some HCPs may be comfortable allowing certain women to be induced, while closely monitoring the use of certain methods to avoid excessive uterine activity.
Induction Success Rate
When factors are favorable (ripe cervix, progressive dilation), most women will deliver within 24 hours, while others may deliver within 2 to 3 days. This time difference is primarily the result of cervical readiness, which – if the cervix is unfavorable – can take anywhere from a few hours to 24 hours to become favorable prior to progressing to augmentation methods.
There are many factors that vary in any specific woman and pregnancy that can affect the success rate of an induction, which include:
Maternal age (debated)
Maternal weight (higher BMI may decrease success rate*)
Number of prior deliveries
Cervical readiness (Bishop score)
Method used and a woman’s response to the method
Whether membranes have ruptured
*In contrast, a study published in September 2021 found that induction of labor in women considered obese reduced the risk of cesarean section compared to women considered obese who were not induced at 39 weeks.
In the event the induction is deemed “unsuccessful” (not well-defined, and could be a general failure to progress), and depending on when this determination is made, a woman can choose to have a cesarean section or try a second induction attempt within a few days, depending on numerous factors.
Prior to an induction, an HCP will evaluate the above factors to determine the likelihood an induction will be successful – on an individualized basis. The HCP will assess the overall health of the mother and baby, as well as whether the cervix is considered favorable, and whether a cervical ripening agent needs to be used first.
Cervical Assessment (and Bishop Score)
If the cervix is favorable (soft, ripe, possibly dilated), the induction process may begin with either a membrane sweep/strip, amniotomy (purposeful membrane rupture), or oxytocin.
These methods are also used for labor augmentation – methods that boost or speed up the progress of labor (that has already started) using specific techniques or procedures. The purpose of augmentation is to avoid a "slowing down" of labor, which may lead to a cesarean section.
If the cervix is unfavorable (firm, closed), a cervical ripening agent must be used prior to the implementation of any augmentation method. If oxytocin is started before the cervix is ready for labor, this would greatly increase the chances of an unsuccessful induction, ultimately resulting in a cesarean section.
Cervical “favorability” is assessed through the Bishop score. The Bishop score is used to predict whether induction will be successful based on various physical characteristics of the cervix/fetus. The higher the score, the higher the probability of success.
In the 1960s, Dr. Edward Bishop developed a scoring system that would result in a range of 0 to 13 points:
Dilation (0 to 3 cm): 0 to 3 points
Effacement (0 to 80%): 0 to 3 points
Consistency (firm, medium, soft): 0 to 2 points
Position (posterior, mid, anterior): 0 to 2 points
Fetal Station (-3 to +2): 0 to 3 points
When the Bishop score is less than 6, it is recommended that a cervical ripening agent be used before labor induction/augmentation methods.
Although the Bishop score is an established method to assess cervical readiness, it is not perfect. It was established prior to modern statistical methods, does not assess general risk of cesarean section, and does not take prior deliveries into account; therefore, other methods have been introduced as more effective options.
Further criticism indicates that cervical assessment is subjective, and one HCP to another may evaluate the same cervix and come up with different estimates. Some researchers believe the Bishop score should no longer be used to predict induction outcome.
A revised, simplified version of the score has been offered which only considers dilation, station and effacement, and appears to have the same or better accuracy at determining induction success. However, additional studies have not been able to determine this method should officially replace the current Bishop score and more research is necessary.
The use of transvaginal ultrasound has also been theorized to have good predictive value based on visual assessment of the cervix, but this method has not yet been revealed to be a good predictor as well, even if it became widely available.
Cervical Ripening Methods
Cervical ripening is considered a mandatory step for women considered to have an "unfavorable" cervix prior to induction.
The various methods for cervical ripening include mechanical and pharmacological (medications). Mechanical methods have been used longer than pharmacological, and although they are still used today, medications are used more often.
Although a membrane sweep can be used as early as 39 weeks and be performed in a doctor’s office during a routine exam, the cervix must also be favorable, or the HCP cannot perform the sweep.
Both mechanical and pharmacological methods are considered very effective, but in general, comparisons between all methods is difficult to perform because of the many different variables involved.
The determination of whether to use medications or different mechanical methods will be a decision made between a woman and her HCP, after a joint discussion of the risks and benefits of each method in the context of that woman’s individual pregnancy.
Prostaglandins are known to regulate the female reproductive system and are involved in the control of ovulation, the menstrual cycle, and induction of labor.
Prostaglandins play a large role in ripening the cervix prior to dilation and effacement by dissolving cervical collagen to encourage the cervix to soften and stretch.
Prostaglandins are important facilitators of uterine activity, and it’s been argued they have an even more important role in labor than oxytocin. Oxytocin is not a prostaglandin, but it can increase the production of prostaglandins; both are required for successful labor and delivery, and both can be made naturally as well as pharmaceutically. The pharmaceutical versions are used in women whose bodies do not produce enough of one or both.
The different types of pharmaceutical versions and mechanical methods used for cervical ripening can be read here.
Membrane sweeping (or stripping) has been associated with an increased chance of labor starting on its own (within 48 hours), after the technique has been completed.
Membrane sweeping involves the HCP placing a finger through the cervix and rotating it several times to separate the membranes from the uterine wall (the richest source of prostaglandins) and internal os of the cervix, to which it is attached. The purpose is to potentially initiate the start of labor, or boost labor that has already started. The cervix must be dilated enough to allow a finger through it.
An amniotomy is the purposeful rupture of membranes (“breaking of water”) by the HCP during labor. An amniotomy can be used to help speed up the progression of labor, usually after 4 cm of dilation, or at full dilation if the membranes have not yet ruptured by then. When used to augment labor, an amniotomy usually indicates a commitment to delivery, as the risk of infection rises (as opposed to going home and trying a second potential induction a few days later).
To perform an amniotomy, the HCP makes a small opening in the amniotic sac with a plastic hook. Women will feel a warm flush of fluid as the sac opens. Infection risk climbs beginning around six hours after the membranes have ruptured if delivery has not yet occurred.
Although the natural, physiological initiation of labor is not completely understood, at least one step includes the release of oxytocin by the pituitary gland which initiates contractions in the uterus.
Oxytocin ("quick birth") is a hormone, and therefore it needs a docking place – a receptor in the organ it is going to affect. As pregnancy progresses, the number of oxytocin receptors in the uterus increases by 100-fold at 32 weeks and by 300-fold near the onset of labor, compared to early pregnancy.
Some women may not experience this surge in receptors, and others may even lose receptors during labor, especially during lengthy inductions. This can cause a failure to either start or progress labor, or respond to administered oxytocin, which would ultimately result in cesarean delivery.
The administration of synthetic oxytocin (Pitocin®/Sintocinon®) during induction or augmentation of labor is very common, has been used for decades, and is very effective.
Oxytocin is used:
After a cervical ripening agent has been successful in women with an unfavorable cervix (and contractions have not started)
Immediately in women who were determined to have a favorable cervix (and contractions have not started)
In women who spontaneously started labor and are experiencing contractions, but contractions have either slowed down or are not building in strength or intensity needed for labor to progress (very subjective, see Dilation Rates.)
There is wide disparity in the current practice of oxytocin augmentation between countries and between hospitals in the same country, to include when its used and the dosage used.
Current intravenous (IV) regimens usually involve incremental increases in dosage. Lower-dose regimens typically involve 0.5 to 2.0 milliunits (mU)/minute as starting doses, with incremental increases of 1.0 to 2.0 mU/minute every 15 to 60 minutes. Lower dose regimens may take longer to delivery than higher dosages, but decreases the risk of excess uterine activity (uterine tachysystole) and may be more tolerable for women. Contractions usually start about 30 minutes after oxytocin is given.
The maximum dose a woman may need and the minimum amount of time to achieve adequate contractions is different in all women, based on preexisting uterine activity, cervical status, receptor status (described above), pregnancy duration, as well as biological factors.
If there is no adequate labor pattern after about 12 hours of oxytocin administration, the induction may be considered as "failed" and administration may be stopped.
To avoid excessive uterine activity, administration is stopped if the number of contractions persists with a frequency of more than five in a 10-minute period, more than seven in a 15-minute period, or with a persistent non-reassuring fetal heart rate pattern. Stopping administration nearly always rapidly lowers contraction frequency, as oxytocin only has a half-life of approximately 3 to 5 minutes.
Oxytocin may cause excess uterine activity as stated above, especially in settings where equipment is not available to titrate doses accurately and/or monitor contractions. Excess uterine activity can stress the baby and increase the risk of uterine rupture, especially in women with a prior cesarean section. Uterine rupture is a life-threatening event in which the uterus tears open along the scar line of a prior cesarean or uterine surgery.
However, uterine rupture associated with oxytocin use is very rare, even in women who have had multiple pregnancies. For example, in a study of 48,718 previously pregnant women, there were eight instances of uterine rupture in general, but only one was due to oxytocin. A second study indicated a rate of 14 uterine ruptures due to oxytocin use in 95,000 births.
Starting at a lower dose, using minimal dosage increments, and monitoring fetal heart rate and contractions, can show early signs of potential complications and the oxytocin administration can be stopped at any time if there is any concern of excess uterine activity.
Labors that are induced have been reported to be more painful than labors that start on their own, leading to the higher likelihood an epidural is requested. However, gradual dosage of oxytocin aims to help women slowly become adjusted to contraction pain (although women respond differently). Further, the same pain relief options are available for women undergoing an induction as those who started labor spontaneously.
Labor induction increases the risk that the uterine muscles may not contract properly after delivery to control bleeding (uterine atony). HCPs can combat this with more oxytocin and manual massage of the uterus. Surgery may be required in severe cases.
Infection can occur with an induction, especially if the induction is prolonged and membranes ruptured early in the process.
Cord prolapse is a risk of amniotomy. When amniotomy is performed, the gush of fluid can potentially cause the umbilical cord to drop down through the cervix which could get compressed during delivery, cutting off oxygen to the baby. This complication can also occur in labors that began spontaneously.
The risk of cesarean section, assisted delivery, and failure to progress/failed induction is inconsistent and unclear. The success of labor induction is determined by many variables, the strongest being cervical favorability. Induction is very effective when a favorable cervix is present prior to augmentation methods.
Additional factors include age of the mother, prior deliveries, the dosage of oxytocin, method of cervical ripening, and whether amniotomy, membrane sweep, or an epidural were used. Additionally, “failed induction” could also be, in some circumstances, not necessarily a failure of the induction process itself (where labor does not start at all), but a general failure to progress with labor.
Women should read Prostaglandins as an immediate follow on to this page, which describes in detail the different cervical ripening agents used prior to induction.
Some hospitals/birthing centers may only offer one type of induction method or one type of cervical ripening medication. Women should ask their HCP/facility what options are available.
Women should ask their HCP or health care facility about their standard oxytocin dosage rate; some hospitals can start much higher and have larger incremental doses than stated above.
Is there a way to determine prior to labor and delivery how a woman will respond to oxytocin?
If a woman failed to respond well to oxytocin during a first pregnancy, is it likely she will respond the same in a subsequent pregnancy or could her response be different?
Practice Bulletin: Induction of Labor (American College of Obstetricians and Gynecologists)
FAQ: Induction of Labor (American College of Obstetricians and Gynecologists)
Induction of Labor at 39 weeks (American College of Obstetricians and Gynecologists)
Clinical Guidance for Integration of the Findings of The ARRIVE Trial: Labor Induction Versus Expectant Management in Low-Risk Nulliparous Women (American College of Obstetricians and Gynecologists)
Induction of Labor (National Institute for Health and Care Excellence - United Kingdom)
Bishop Score Calculator (Perinatology.com)
This is an excellent video describing the induction process and the many different options and factors that depend on a safe, successful induction.