Inducing Labor Past 39 Weeks Does Not Increase Your Chances of Having a C-Section

Pregnant women often fear that having their labor induced will lead to a c-section. In her bestselling book, Expecting Better, Emily Oster cites fear of a c-section as the primary reason she opted not to have her labor induced. Other reputable sources like the Mayo Clinic Online and March of Dimes claim that inductions increase the odds of a c-section.

But, according to the latest scientific research, these fears are unfounded. Past 39 weeks, labor inductions do not appear to raise the risk of having a c-section. Instead, being induced lowers your chances of a c-section by about 20%.

How did we get this so wrong? Until about 5 years ago, almost all the evidence on inductions and c-sections came from observational studies, which were all subtly biased, because they compared women who went into labor on their own with women who were induced.

What was wrong with this comparison?

Observational studies typically control for gestational age. This means that, in effect, they match women by the week they delivered, because the risks associated with giving birth climb as pregnancies continue past 39 weeks. Women who went into labor spontaneously at 39 weeks were compared to women who were induced at 39 weeks, and so on.

And, when matched by delivery week, women who go into labor spontaneously are less likely to have c-sections.

So what’s the problem? Pregnancies that go into labor spontaneously by a certain week are different from those that don’t.

Think about it this way: What is choice women actually face past 39 weeks? Women cannot will themselves into labor, or there would be a lot fewer pregnancies going past 40 weeks. Instead, pregnant women can only choose either to be induced or to wait it out.

So the proper study design compares these two options: being induced versus waiting it out. This is the approach taken by recent randomized controlled trials on inductions.

In these trials, pregnant women are randomly assigned to be induced past a certain point in pregnancy, for example, at 41 weeks, or to wait it out. Women assigned to wait may go into labor on their own or be induced at a later date. Using this approach, studies find, almost without exception, that relative to waiting and delivering at a later point in pregnancy, inducing labor leads to a lower chance of a c-section.

A 2009 meta-analysis (combined analysis of multiple studies) was the first to convincingly reveal the problem.

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The analysis compared the results of observational studies and randomized controlled trials (RCTs). The observational studies found a roughly 20% increase in the risk of c-section following inductions; the better-designed RCTs found a roughly 20% reduction in the risk.

Subsequent large meta-analyses (see here, here, and here) have all confirmed the 2009 analysis, unanimously finding that labor reduces the risk of a c-section by 10-20%. This appears to hold pretty much across the board, for women with “ripe” and “unripe” cervixes, and for high- and low-risk pregnancies.

There is one important caveat, though. Most of the randomized controlled trials have focused on “late term” (41+ weeks) and post-term (42+ weeks) pregnancies. Fewer studies have addressed inducing between 39-41 weeks. The available evidence indicates that inductions lower the chances of a c-section between 39-41 weeks, but the case is not as strong.

(As an aside, bias may not totally explain the difference in findings between RCTs and observational studies. Because most observational studies are older than the RCTs, recent changes in how labor is induced may also play a role. Prostaglandins are now routinely used to ripen the cervix prior to an induction. This practice lowers the chances of a c-section, compared to inducing labor with pitocin alone.)

Why do inductions lower the risk of a c-section?

Once a pregnancy is full term, the chances of a c-section rise with each passing week, and this increase in risk is larger than the benefit of spontaneous labor.

The two “true” reasons for a c-section–a head that is too large to fit through the birth canal and placental failure–become more common late in pregnancy. Other complications, like gestational hypertension, pre-eclampsia, and low levels of amniotic fluid, also become more common past 40 weeks.

Part of this rise is probably also because doctors treat women who are “post-dates” differently.  Doctors may be more anxious when delivering late or post-term pregnancies. In the midst of a stressful delivery, additional anxiety could tip the balance towards a c-section.

Past 39 weeks, if you are hoping to avoid a c-section, is it better to be induced than to wait for labor to occur on its own?

Even though the risk of a c-section increases past 39 weeks, just when to opt for an induction is far from clear. An observational study of over a million women living in Scotland illustrates the problem. Aware of the problems plaguing previous observational studies, the Scottish researchers conducted their analyses in two ways. First, mimicking an RCT, they compared women who were induced with women who delivered in a later pregnancy week. Second, they compared women who were induced with women who delivered the same pregnancy week or later. Compared with women who delivered in a later week, women who were induced had a lower chance of a c-section. But compared to women who delivered in the same or a later week, women who were induced had a slightly higher chance of a c-section.

In essence, if labor is just around the corner and your primary goal is to avoid a c-section, it is better to go into labor on your own. Of course, until we can do a better job predicting the onset of labor, this is not very helpful advice.

Once you reach 39 weeks, is it better overall to deliver now rather than later?

Some doctors have proposed that past 39 weeks, it is better to induce labor than to wait for it to occur spontaneously. The risks of stillbirth and of the fetus inhaling meconium rise late in pregnancy. By cutting pregnancies short, inductions lower these risks.

This is the main reason why inductions are now routinely offered to women past 41 weeks. Some doctors have taken this argument a step further, arguing that a widespread practice of scheduled c-sections at 39 weeks would greatly reduce the number of stillbirths.

(Note: The benefit of early delivery in terms of perinatal death, in relative terms, is fairly large. But because fetal death is a rare event in the developed world, in absolute terms, the survival benefit is small. To avoid a single stillbirth, doctors would need to perform an estimated 410 inductions.)

a newborn baby

Personally, I am not convinced by these arguments. Somewhere between 30-40% of pregnancies would naturally continue past 40 weeks. That’s almost half of all pregnancies! Cutting these “overdue” pregnancies short, whatever the apparent benefits, feels wrong.

I worry about unforeseen negative consequences. We still do not understand why some pregnancies last longer than others, nor the exact biological processes that initiate labor. The research on term and postterm inductions, while substantial, has focused primarily a small number of outcomes, like c-sections, fetal deaths, and NICU admissions, because these are easily obtained through medical records.

But what about the long-term effects of inducing labor on overall health and brain development? We do not really know what they might be. These outcomes are difficult to assess and time-consuming to collect. Almost no studies have investigated them.

And it was just these kinds of subtle long-term negative effects that recently led the American College of Obstetricians and Gynecologists (ACOG) to recommend against elective inductions before 39 weeks of pregnancy. Many pregnancies are inaccurately dated. So even inductions scheduled at 39 weeks could potentially truncate some pregnancies earlier than recommended, at 37 or 38 weeks.

To me, these are all reasons to tread cautiously when inducing labor.

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Author: Amy Kiefer

I am a research scientist and mother of three munchkins. I live in the Bay Area and when I am not child wrangling and catching up on lost sleep, I love digging into the research on all things fertility, pregnancy, and breastfeeding related.

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