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Studies on Postdates & Induction

Updated: Feb 13, 2022

I had a homework assignment in my beginner midwifery program to find studies relating to postdates and induction. In my experience, obstetricians in general do not often provide evidence-based care. I wanted to share my findings with you so you can make your own informed choices. I analyzed 6 studies, which you'll see below, separated by dividers.

The term "postdates" refers to 42 weeks gestation and beyond.

#1 Study: ARRIVE 2019

I wanted to address this study even though it’s not directly related to postdates because it seems to be the biggest permission slip for care providers to start pressuring pregnant mothers into induction any time between 39-41 weeks. I rarely ever hear of a woman being “allowed” to go to 42 weeks anymore. They have stated reasons like: potential big baby getting stuck, placenta not functioning as well, amniotic fluid will get too low, higher c-section rate, stillbirth, etc.

Official Title: Induction in Nulliparous Women at 39 Weeks to Prevent Adverse Outcomes: A Randomized Controlled Trial


The ARRIVE study is a randomized control trial aiming to confirm that elective induction at 39 weeks could improve infant and maternal outcomes.

Sponsors and Collaborators:

  • The George Washington University Biostatistics Center

  • Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD)

How it was administered:

  • 6,106 participants (out of 22,533 who were eligible) from 41 hospitals in the United States

  • Randomly assigned to one of two groups: elective induction of labor between 39 weeks 0 days and 39 weeks 4 days; or expectant management (unless a medical indication arises) until at least 40 weeks 5 days

  • Those who are selected for labor induction will be administered synthetic oxytocin through an IV line

  • Those who are selected for labor induction but whom have an “unfavorable cervix” (Bishop score less than 5), will first undergo cervical ripening with methods left to the discretion of the patient’s physician (usually Cervidil or Cytotec)

  • Inclusion criteria: no previous pregnancy beyond 20 weeks, singleton gestation (one baby, not twins), gestational age at randomization between 38 weeks 0 days and 38 weeks 6 days

  • See study for exclusion criteria. They are basically only accepting participants who are deemed low risk.

  • The primary outcome was a composite of perinatal death or severe neonatal complications; the principal secondary outcome was cesarean delivery.


  • Perinatal death or severe complications occurred in 4.3% of babies in the induction group and 5.4% in the expectant management group. This is deemed an insignificant difference.

  • The frequency of cesarean delivery resulted in 18.6% in the induction group and 22.2% in the expectant management group. This is deemed a significant difference.

My analysis:

Yes, if you’re going to force babies out at 39 weeks versus nearly 41 weeks, the earlier gestation will mostly be smaller and therefore may be easier to fit through the pelvis. Lower c-section rate may be achieved, but at what cost? For example, they don’t mention the higher likelihood of getting an epidural and the risks that come with that. There are plenty of other ways to lower your risk of c-section without increasing your risk of interventions. Very good!

I think it is important to note here that the study is comparing induction at 39 weeks versus induction at 40 weeks 5 days. It is not comparing labor induction to spontaneous labor. YES!

We know that the process of spontaneous labor occurs by the natural release of oxytocin from the mother’s brain. Oxytocin is a hormone that is blocked when the sympathetic nervous system is activated and catecholamines (stress hormones) are released.

Labor induced in a hospital setting inherently involves new and unfamiliar medical procedures, mechanically produced uterine contractions, lack of mobility for the mother due to continuous fetal heart rate monitors and IV drip, all of which are likely to cause a release of catecholamines, inhibiting the spontaneous flow of labor. And too, induction meds block the release of oxytocin, so mothers do not get the psycho-emotional benefits of oxytocin.

Thus, more medical interventions will be used to facilitate labor. This is what we call the cascade of interventions, which often leads to c-section, perineal tearing, pelvic floor damage, manual (forceps or vacuum) delivery and risk of brain damage to the infant, separation of mother and infant post birth, low milk supply, difficulty bonding, and postpartum depression.

A point also made by Rebecca Decker from Evidence Based Birth -

“Most of the women in this study were cared for by physicians (94%). Studies on out-of-hospital birth show that midwives achieve extremely low rates of Cesarean without the regular use of elective inductions. In the U.S., the Cesarean rate is about 5% at planned home births and 6% at midwifery-led birth centers (Cheyney et al. 2014; Stapleton et al. 2013). Hospitals with a higher percentage of midwife-attended births also tend to have lower rates of Cesarean; a recent study found a 15% Cesarean rate for hospitals that had more than 40% of their births attended by midwives (Attanasio and Kozhimannil, 2018).”

An important note from ACOG:

“They urge care providers to first consider three important factors: the values and preferences of the pregnant woman, the staffing and facility resources available (to assist longer labors), and the protocol for “failed” induction.”



#2 Study:

Induction of labor as compared with serial antenatal monitoring in post-term pregnancy. A randomized controlled trial. The Canadian Multicenter Post-term Pregnancy Trial Group


The goal of this study is to determine whether the induction of labor results in better outcomes than expectant management of serial fetal monitoring while awaiting spontaneous labor.

Sponsors and Collaborators:

  • Department of Obstetrics and Gynecology, Women's College Hospital, University of Toronto, Canada

How it was administered:

  • 3407 women with uncomplicated pregnancies of 41 or more weeks’ duration in 22 hospitals throughout Canada

  • Randomly assigned to undergo induction of labor or to have serial antenatal monitoring and spontaneous labor (unless there was evidence of fetal or maternal compromise)

  • In the induction group, labor was induced by the intracervical application of prostaglandin E2. If the gel was not used or if it did not induce labor, labor was induced by IV oxytocin, amniotomy, or both.

  • Serial antenatal monitoring consisted of fetal kick counts, nonstress tests, and assessments of amniotic fluid volume

  • Women were excluded from the study if the cervix was dilated ≥3cm, if the gestational age was ≥44 weeks, if the presentation of the fetus was noncephalic, or if there was evidence of a lethal congenital anomaly, maternal diabetes mellitus, preeclampsia, IUGR, or PROM. See study for more exclusions.

  • The outcomes measured were the rates of perinatal mortality, neonatal morbidity, and delivery by cesarean section


  • Among the 1701 women in the induction group, 360 (21.2%) underwent cesarean section, as compared with 418 (24.5%) of the 1706 women in the monitoring group

  • When two infants with lethal congenital anomalies were excluded, there were no perinatal deaths in the induction group and two stillbirths in the monitoring group. The frequency of neonatal morbidity was similar in the two groups, deemed not significant.

  • Perinatal Mortality and Neonatal Morbidity - “There were two stillbirths in the monitoring group and none in the induction group. In the case of the first stillbirth, a 28-year-old woman with two previous live-born children began fetal-movement counts on the day of randomization. Three days later the intrauterine death of the fetus was confirmed. Meconium staining of the membranes was noted at delivery. The stillborn infant weighed 3175 g at delivery. An autopsy revealed hypoxic—ischemic encephalopathy. In the second case, a 31-year-old woman in her first pregnancy had normal results on a nonstress test and a normal amniotic-fluid—volume assessment on the day of randomization; antenatal testing continued until labor began spontaneously five days later. Acute fetal distress was diagnosed, and an emergency cesarean section was performed. The stillborn infant, who could not be resuscitated, weighed 2600 g. The autopsy revealed massive aspiration of meconium.” (likely a cord accident)

My Analysis:

This study compared a group undergoing induction at or shortly after 41 weeks to a group undergoing expectant management. But the expectant management group would be induced at 44 weeks if they were still pregnant.

It’s very possible that the two neonatal deaths could have been from random causes, not necessarily caused by gestational age. The actual gestational age of both babies that died is not known. In the first noted case, the death occurred 3 days after randomization. In the second noted case, the death occurred 5 days after randomization. But the participants of the study could have been 41 weeks or more. We don’t know exactly.

I find it interesting that in both studies I’ve looked at so far, the cesarean rate is lower in the induction group. I want to note that this doesn’t take into account the biases of the doctors and their personal anxiety dealing with later term births, and the fact that most of the participants are not working with midwives. Midwives both in and out of the hospital have much lower cesarean rates than physician attended births.



#3 Study:

Postterm Pregnancy


This seems to be some sort of meta study, looking at many different studies. It states, “Postterm pregnancy is a pregnancy that extends to 42 weeks of gestation or beyond… The perinatal mortality rate, defined as stillbirths plus early neonatal deaths, at 42 weeks of gestation is twice as high as that at term (4-7 versus 2-3 per 1000 deliveries, respectively). It increases 4-fold at 43 weeks and 5-7-fold at 44 weeks (Bakketeig and Bergsjo, 1989; Feldman, 1992; Hilder et al., 1998; Cotzias et al., 1999). These data also demonstrate that when calculated per 1000 ongoing pregnancies, fetal and neonatal mortality rates increase sharply after 40 weeks (Hilder et al., 1998??) (Fig. 1). It is believed that utero-placental insufficiency, meconium aspiration and intrauterine infection are the underlying causes of the increased perinatal mortality rates in these cases (Hannah, 1993).”

My Analysis:

I keep seeing these studies trying to figure out the best way to keep mothers and babies alive. But I was curious why they kept saying that essentially it becomes more dangerous the longer the pregnancy lasts. So I looked for answers, and this is what I found.

It may be true that the rate of fetal and neonatal mortality increases as pregnancy goes on. But safety and risk are relative, in my opinion. Looking at the text above stating that the perinatal mortality rate is twice as high at 42 weeks versus at term (0.4%-0.7% vs. 0.2%-0.3%), both could be considered pretty low. There is always a risk of neonatal death at any stage of pregnancy. Life is inherently risky. The real question is, what risks are you comfortable with?

Also consider common risks of hospital birth (infection, hypoxia due to induction/Pitocin, mismanagement of shoulder dystocia, mismanagement of third stage, and all the factors mentioned in analyzing the previous studies). Bottom line…not enough information on controls in each study to draw any conclusion.

For example, I’m more comfortable with the risk of being farther away from an operating room during birth because I have a much lower risk of having a cesarean by staying at home.



#4 Study:

Risk of cesarean delivery after induction at term in nulliparous women with an unfavorable cervix

Objective: The purpose of this study was to evaluate the effect of induction on the route of delivery in nulliparous women laboring at term in a community hospital system.

Study design: From April 1997 to October 1999, there were 7282 deliveries in nulliparous patients who met inclusion criteria. Cesarean delivery rates were calculated for patients in spontaneous labor and for patients who underwent induction.

Results: Among 4635 women (63.7%) in spontaneous labor, the cesarean delivery rate was 11.5% versus 23.7% among the 2647 (36.3%) patients who underwent induction. An important variable that affected the delivery route was the Bishop score at the initiation of the induction. The cesarean delivery rate was 31.5% among patients whose Bishop score was <5 at induction versus 18.1% for patients with a score > or =5(P <.001).

Conclusion: The induction of labor in nulliparous patients, especially those women with an unfavorable cervix as measured by Bishop score, is associated with a significantly increased risk of cesarean delivery.

My Analysis:

I couldn’t find any more details about this study but it looks interesting because the data shows more of what I’ve witnessed in hospitals as a doula.

It looks like a retrospective study, looking back over 2.5 years. And I believe it’s only looking at one hospital. But it is a decent sample size.

What was most interesting here is that there’s a much bigger difference in cesarean rate between the two groups (spontaneous labor versus induction) compared to the first two studies I looked at. It’s not surprising to me that Bishop’s score was a key factor. If a cervix isn’t showing signs of being ripe and ready to open, the baby and mother’s body are not likely ready for labor. Therefore, it can take a lot of force/intervention to make something happen.



Study #5:

Elective induction of labor: failure to follow guidelines and risk of cesarean delivery


Estimate the frequency of failure to follow the French consensus guidelines for elective induction, and assess how failure affects the rate of cesarean delivery.

Study design:

  • Compared cesarean rates according to mode of onset of labor among 5,046 low-risk patients

  • Violation of the guidelines was defined as induction before 38 weeks or with a Bishop score <5 or with prostaglandins.

  • The cesarean risk was analysed with a bivariable and then a multivariable analysis, which used a multilevel logistic model.


  • Women with electively induced and spontaneous labor had identical cesarean rates (4.1%).

  • The guidelines were not followed in 23.2% of elective inductions.

  • The risk of cesarean was higher after induction with a Bishop score <5, than after spontaneous labor

  • Elective induction with a favourable cervix did not increase the cesarean risk

  • In nulliparas, failure to follow the guidelines tripled the risk of cesarean

  • On the other hand, elective induction of labor for women with a favourable cervix did not increase the risk of cesarean over the risk with spontaneous labor

My analysis:

This study was held in France where, similarly to the United States, the induction rate has increased tremendously in the last couple decades. In both countries, the cesarean rate has also increased. Might this be correlated? The rates could have also simultaneously increased due to lawsuits, profits of intervention, more surgical skill, technological abilities, etc.

This was a prospective, observational study spanning over 138 maternity units.

It’s pretty clear from the results that there isn’t much of a difference in cesarean rates comparing spontaneous labor and induction with a ripe cervix with a Bishop’s score of 5 or more. But cesarean rate quadrupled when induction was carried out against guidelines compared to spontaneous labor. Results seem to show that one of the most important factors in successful vaginal delivery is cervical ripeness at onset of induction.



Study #6: 2012

The CIC Trial - castor oil for induction of contractions in post-term pregnancies

Sponsors and Collaborators:

  • Hadassah University Hospital, Mount Scopus, Obstetrics and Gynecology, Jerusalem, Israel

  • Hebrew University-Hadassah, Braun School of Public Health, Jerusalem, Israel


To study the efficacy and safety of castor oil for induction of labor in post date pregnancies

Study design:

  • A prospective, randomized, double blind and placebo controlled study was conducted

  • Criteria for eligibility were singleton pregnancy, 40-42 weeks, Bishop Score ≤7, no uterine activity and no previous cesarean section

  • Treatment and control groups received 60ml of castor oil and placebo orally, respectively

  • Logistic regression was used to compare the odds of entering the active phase of delivery within 12 hours of administration of intervention between the two groups.

  • Kaplan-Meier analysis (Survival analysis) was performed to compare the time from administration of intervention to entering the active phase of labor


  • The odds of entering active phase of labor within 12 hours of administration of intervention was 3 times higher among women receiving castor oil compared to women receiving placebo

  • No differences between the two groups were found in labor complications and neonatal outcomes

My Analysis:

Though it is a small sample size, it is double blind and placebo controlled. That shows me I can trust conclusions. Along with the clear efficiency of castor oil in inducing labor, there doesn’t seem to be an increase in labor complications or negative effects on neonatal outcomes.

This sounds promising to me and a valid choice for mothers who are interested in inducing their labor outside of a hospital setting.


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