Induction Math: The Importance of the Bishop Score

Image from Relay Health, as displayed on http://www.med.umich.edu/

Yet another study shows that inducing labor increases the risk for a cesarean, especially in first-time mothers with an unripe cervix (one that has not done much dilating and effacing yet).

In the study, they specifically looked at cesarean rates among induced mothers with an unripe cervix.  They found that cesarean rates were particularly high among first-time mothers who were induced with an unripe cervix, but the risk was also elevated for mothers with prior vaginal births who were induced on an unripe cervix.  

This is why it is absolutely vital to ask your provider about your Bishop Score before agreeing to induce labor.

The Bishop Score

The Bishop Score is a measure of how soft and ripe your cervix is before labor. It can help predict whether or not your body is ready for labor, and whether or not an induction is likely to succeed or fail.

A vaginal exam is done and the care provider evaluates the degree of:

• cervical dilation (how far the cervix has opened so far)
• cervical effacement (how thinned out the cervical walls are)
• cervical consistency (how soft or firm the cervix is)
• cervical position (whether the cervix is pointing forwards or backwards relative to the vaginal walls)
• fetal station (how far down the baby is in the pelvis)

[If you need further explanation about the terminology above, you can find an excellent explanation and great illustrations here.]

Each factor is "graded" on a scale of 0-2 or 0-3.  The maximum possible score is 13. This all adds up to what some doulas call "induction math".

The exact cut-offs used differs by source, but generally a score of 5 or less indicates the woman is unlikely to go into labor spontaneously at that time, and that an induction is likely to fail (result in a cesarean).

A score of 8 or more indicates that an induction is more likely to succeed.  A score of 9 or more indicates the woman will likely go into labor on her own very soon.

Sometimes care providers use modifications of a Bishop Score to help predict likelihood of successful induction.  One point may be added to the score for the existence of pre-eclampsia or for every previous vaginal birth.  One point is often subtracted for a "postdates" pregnancy, being a first-time mother (or for no previous vaginal births), or preterm prelabor rupture of membranes.

The Bishop Score is just one tool for predicting a woman's response to induction.  Obviously, other factors matter as well.

The Influence of Fetal Position

One often-overlooked factor is fetal position ─ which way a head-down baby is facing in utero.  Most babies face either occiput anterior (back of the head towards mother's belly; baby looking at mother's back) or occiput posterior (back of the head towards mother's back; baby looking at mother's belly).

Often a low Bishop Score is associated with a posterior baby, which is a less ideal position for birth.  In this position, the the baby's head is not putting pressure as efficiently on the cervix, so there is less cervical effacement or dilation, and the cervix is often posterior (pointed towards the lower vaginal wall and hard to reach during a vaginal exam).  When the baby rotates to anterior, the Bishop Score often changes dramatically because the physics of the baby's pressure changes.

Many "overdue" babies are actually posterior babies whose positions are simply not putting the most efficient kind of pressure on the cervix to thin and dilate, and so the body wisely does not go into labor yet.  Forcing the issue by inducing labor when the baby is posterior (or has other malpositions) tends to result in long, hard labors that often end in a c-section.

[This is a pet peeve for me, because it is the story of my first c-section....induced at 40 weeks despite a very low Bishop Score (the OB told me I had a "horrible" cervix) and a malpositioned baby.  Small wonder I ended up with a cesarean!]

So if your Bishop Score is low, it may not just be that your body is not "ripe" for labor, it may also be that the reason you are not ripe is because the baby is not in a great position for labor.  For that reason, it may be wise to delay inducing until the baby is in a more favorable position for labor.

Seeing a pregnancy chiropractor and getting adjusted may help encourage the baby to get into a better position for labor, which in turn might lessen your chances for a cesarean.  (This is what was key for me in my VBACs.)

Induction Triples the Risk for Cesarean in First-Time Mothers

In the 2011 study listed below, the charts of women who were induced with a Bishop score of less than 7 were studied.

Those first-time moms who were induced with a Bishop score less than 7 had a whopping 42% cesarean rate.  

This shows just how important it is to have a nice ripe cervix before inducing labor, and especially so in first-time moms, whose cervices have never dilated before.

Sometimes a doctor will try to reassure you that it doesn't matter if your cervix isn't ripe; they use drugs that help ripen the cervix before starting the induction drugs.  And it's true that these drugs can help at times.

However, even with cervical ripening methods, many inductions still fail.  Cervical ripening drugs are simply not a panacea for preparing the body for birth when it's not ready.  They work best when the baby is in good position and the body is close to being ready but not quite there yet.  Cervical ripening methods are very unlikely to help in women with very low Bishop Scores.

Now, if the cervix has dilated before, induction is more likely to work. Women who had had vaginal births before had a much lower cesarean rate after induction in this study, 14%.  Still, this was nearly double the cesarean rate of those multips who went into labor spontaneously.  

Obviously, cervical ripeness matters, even in women who have had vaginal births before.

Take-Home Points About Induction

Additional research confirms that cervical ripeness is one of the key factors in whether or not an induction will work.  Here are some take-home lessons from the 2011 study:

• Whenever possible, wait to go into labor spontaneously
• If induction is being considered, try to avoid inducing if your Bishop Score is less than 7

Other general bits of wisdom about inductions:

• Try to see a provider who is not induction-happy and won't induce automatically at a certain gestation.  This is especially important for women of size, because many providers induce "obese" women at extremely high rates (a 50-60% induction rate is common in many recent studies).  This is a direct but under-acknowledged factor in the very high cesarean rate in women of size.

• Question whether an induction is truly necessary in your case.  Many inductions are done routinely, simply because it is convenient for the provider or protocol to induce by a certain gestational age.  However, you don't have to agree to this intervention.  Discuss the pros and cons of the induction vs. waiting with the provider, and see if you can negotiate for more time.  

• If you are contemplating an induction, ask about your Bishop Score before agreeing to the induction.  If at all possible, try not to induce before you have a favorable Bishop Score. 

• If you are contemplating induction, ask about your baby's position before agreeing to the induction.   If the baby is not anterior, consider delaying the induction.  Seeing a well-trained pregnancy chiropractor may help encourage the baby to get into an easier position for birth.

• If a medical condition makes induction necessary even with an unripe cervix, look into ways to increase cervical ripeness before the induction.  This can include acupuncture, herbs, and cervical ripening agents.  Consider the pros and cons of each choice carefully for your situation, and remember that gentler methods generally need a longer time to be effective.  Don't wait to the last minute to try the more gentle methods of cervical ripening if you are very likely to face induction.

• Be sure your provider allows adequate induction time before resorting to a cesarean.  Recent research has shown that many providers move to a cesarean too soon in an induction; allowing just a few more hours (provided mother and baby are doing well ) may result in a vaginal birth after all.

• Remember, induction doesn't automatically mean you're going to have a cesarean; many women who are induced do end up having a vaginal birth.  Go in with a positive attitude, try to remain as mobile as possible, and be sure to have professional labor support (a doula), who can often help maximize your chances even during an induction.  However, because an induction does increase the chances for cesarean, be prepared for any possibility and have a cesarean birth plan ready if one becomes necessary.

Induction is more and more common these days.  This is why it is important for all women to be informed birth consumers and learn more about their choices around induction.  The Bishop Score is an important ─ and often overlooked ─ part of this discussion.

*Thanks to Holistic NYC Doula for the term "induction math" and for her excellent posts on the topic of the Bishop Score.



References

Acta Obstet Gynecol Scand. 2011 Oct;90(10):1094-9. doi: 10.1111/j.1600-0412.2011.01213.x. Epub 2011 Jul 21. Induction of labor and the risk for emergency cesarean section in nulliparous and multiparous women. Thorsell M, et al.   PMID: 21679162

OBJECTIVE: To assess the risk for emergency cesarean section among women in whom labor was induced in gestational week ≥41 and to evaluate if parity and mode of induction affected this association.
DESIGN: Hospital-based retrospective cohort study.
POPULATION: Singleton pregnancies delivered after ≥41 gestational weeks at Danderyd Hospital, Stockholm, Sweden, during 2002-2006.
MATERIAL AND METHODS: Of 23 030 singleton pregnancies meeting the entry criteria, 881 were induced with a Bishop score of less than 7. Obstetric outcome was assessed through linkage with the Swedish Medical Birth Registry and a local obstetrical database containing information from patients' medical files. Results were adjusted for body mass index, age and the use of epidural analgesia.
MAIN OUTCOME MEASURE: Risk for emergency cesarean section.
RESULTS: Among women who were induced, the proportions delivered by emergency cesarean section were 42% for nulliparous and 14% for multiparous. Compared to spontaneous onset, this corresponded to a more than threefold increase in risk for nulliparous women (OR 3.34, 95% CI 2.77-4.04) and an almost twofold increase in risk for multiparous women (OR 1.94, 95% CI 1.24-3.02). There was no significant difference in risk for emergency cesarean section between the two methods of induction (PGE(2) and transcervical catheter).
CONCLUSIONS: Compared to spontaneous onset of delivery, induction of labor is associated with an increased risk for emergency cesarean section both among nulliparous and multiparous women. When labor is induced, the high risk for emergency cesarean must be kept in mind.

Clin Obstet Gynecol. 2006 Sep;49(3):564-72. Preinduction cervical assessment. Baacke KA, Edwards RK.   PMID: 16885663 

The rate of labor induction is increasing in the United States. Methods for quantifying cervical factors have been developed to identify patients who may benefit from cervical ripening before induction. The first cervical scoring systems used digital examination. More recently, cervical ultrasound and testing for the presence of fetal fibronectin have been suggested to evaluatecervical readiness for labor induction, but neither of these methods provides a significant improvement over digital examination. The Bishop score, the most widely used digital examination scoring system, still is the most cost effective and accurate method of evaluating the cervix before labor induction.

Obstet Gynecol. 2005 Apr;105(4):690-7. Bishop score and risk of cesarean delivery after induction of labor in nulliparous women. Vrouenraets FP, et al.  PMID: 15802392 

OBJECTIVE: To quantify the risk and risk factors for cesarean delivery associated with medical and elective induction of labor in nulliparous women.
METHODS: A prospective cohort study was performed in nulliparous women at term with vertex singleton gestations who hadlabor induced at 2 obstetrical centers. Medical and elective indications and Bishop scores were recorded before labor induction. Obstetric and neonatal data were analyzed and compared with the results in women with a spontaneous onset of labor. Data were analyzed using univariate and multivariable regression modeling.
RESULTS: A total of 1,389 women were included in the study. The cesarean delivery rate was 12.0% in women with a spontaneous onset of labor (n = 765), 23.4% in women undergoing labor induction for medical reasons (n = 435) (unadjusted odds ratio [OR] 2.24; 95% confidence interval [CI] 1.64-3.06), and 23.8% in women whose labor was electively induced (n = 189) (unadjusted OR 2.29; 95% CI 1.53-3.41). However, after adjusting for the Bishop score at admission, no significant differences in cesarean delivery rates were found among the 3 groups. A Bishop score of 5 or less was a predominant risk factor for a cesarean delivery in all 3 groups (adjusted OR 2.32; 95% CI 1.66-3.25). Other variables with significantly increased risk for cesarean delivery included maternal age of 30 years or older, body mass index of 31 or higher, use of epidural analgesia during the first stage oflabor, and birth weight of 3,500 g or higher. In both induction groups, more newborns required neonatal care, more mothers needed a blood transfusion, and the maternal hospital stay was longer.
CONCLUSION: Compared with spontaneous onset of labor, medical and elective induction of labor in nulliparous women at term with a single fetus in cephalic presentation is associated with an increased risk of cesarean delivery, predominantly related to an unfavorable Bishop score at admission.