In countries with well-resourced, medicalised maternity services, endemic induction has changed the duration of pregnancy within one generation. For example, in Australia, most babies are born at 39 weeks (31.6%) [1]. In fact, more babies are born at 38 weeks (23%) than at 40 weeks (19.2), and only 0.6% of pregnancies get to 42 weeks or beyond. The majority of babies are born before their due date, and most inductions are not carried out for 'prolonged pregnancy'. Instead, increasing groups of women are being induced at 39 weeks for all kinds of reasons (maternal age, 'gestational diabetes', high BMI, etc.). This post is about the complexities of risk for women who gestate beyond their care provider's comfort zone and are offered an induction for a 'prolonged' pregnancy.

A Quick Word About Risk

There are all kinds of problems associated with providing care based on generalised risk rather than on individual women's needs and preferences. However, risk, along with 'due dates', is here to stay, and women usually want to know about risks. Risk is a very personal concept and different women will consider different risks to be significant to them. Everything we do in life involves risk. So, when considering whether to do X or Y, there is no risk-free option. All women can do is choose the option with the risks they are most willing to take. However, in order to make a decision, women need adequate information about the risks involved in each option. This is a legal requirement for consent, and If a health care provider fails to provide adequate information they could be faced with legal action.

What is a Prolonged Pregnancy?

Before we go any further, let's get some definitions clear:

• Term: a 'normal' and healthy gestation period is between 37 weeks and 42 weeks.
• Post-dates: the pregnancy has continued beyond the decided due (guess) date, i.e. is over 40 weeks.
• Post-term: the pregnancy has continued beyond term, i.e. 42+ weeks.

The definition of a 'prolonged pregnancy' is one that has continued beyond 42 weeks, i.e. is post-term. Induction is usually offered when a pregnancy becomes post-dates or even before, with the aim of preventing it from becoming 'prolonged'. Therefore, very few women experience a prolonged pregnancy.

The idea of a prolonged pregnancy assumes that all women naturally gestate their babies for the same length of time. However, it seems that genetic differences may influence what is a 'normal' gestation length for a particular woman. For example, there appears to be a genetic influence on pregnancy duration from both the mother and father [2]. The length of gestation may also be influenced by how well-nourished a woman is [3].

It is important to remember that the baby is responsible for the preparation and initiation of physiological labour. The baby chooses their birth date, and lots of physical changes occur before labour starts that prepare the woman and baby for birth, breastfeeding and bonding. Induction of labour is often carried out before these changes occur.

Risks Associated with Waiting for Spontaneous Labour

Ageing placenta?

Some people believe that the placenta has a best-before date and starts to deteriorate after 40 weeks, resulting in reduced nutrition and oxygen for the baby. There is evidence that the structure and biochemistry of the placenta change as pregnancy develops. In the final weeks of pregnancy, there is an increase in biomarkers involved in the ageing process of tissues in the body. So, yes, the placenta does show signs of ageing [4]. The question is whether these changes are physiological or pathological. And the answer is that the changes are normal and physiological and likely happen for a reason. For example, in recent years, there has been discussion about how these placental changes may be important for the separation of the placenta post-birth [5]. Another potential purpose for placental changes is to prepare the baby to initiate labour. Cortisol (a stress hormone) has an important role in preparing the baby for labour and extrauterine life. It initiates the maturation of the baby's lungs and vital organs, which happens before the baby signals to the mother they are ready to be born. Perhaps the increased cortisol is a response to placental changes slight reduction in oxygen?

Big baby

People also have concerns that the baby will grow huge and, therefore, be difficult to birth. There is evidence that babies continue to grow bigger the longer they gestate, and this contradicts the above theory about the ageing placenta being insufficient to sustain the baby. If the placenta stops functioning effectively, how does the baby continue to grow so well? Big babies are pretty good at finding their way out of their mother's expandable pelvis. The research about complications relating to big babies suggests that it is the interventions carried out when a baby is assumed to be big, rather than the actual size of the baby, that primarily contributes to complications. See this post for more on big babies.

Meconium-stained amniotic fluid

There is an increased chance that the baby will pass meconium as their bowels mature. This is not a complication, and I have written about meconium-stained amniotic fluid in this post.

Perinatal death

The general rate of perinatal death (stillbirth + newborn death) increases as pregnancies advance beyond term. The rate remains small but is statistically significant. For example, a systematic review and meta-analysis [6] reported that: 'The overall gestation-week-specific prospective risk of stillbirth steadily increased with gestational age, from 0.11 per 1,000 pregnancies at 37 weeks to 3.18 per 1,000 at 42 weeks gestation... The risks of newborn death remained constant between 38 and 41 weeks, and only increased beyond 41 weeks'. When the review looked at data for low-risk pregnancies, the rates of stillbirth were even lower – 0.80 per 1000 at 41 weeks and 0.88 per 1000 at 42 weeks.

It is very difficult to determine whether induction of labour reduces the risk of stillbirth in comparison to spontaneous labour. This is because randomised controlled trials compare induction with 'expectant management' rather than with spontaneous labour. So, what you end up with is induction at a particular gestation (e.g. 41 weeks or 39 weeks) compared with expectant management that includes induction after the particular gestation, induction for complications, and spontaneous labour. A Cochrane Review [7] summarises the research examining induction vs expectant management: 'There is a clear reduction in perinatal death with a policy of labour induction at or beyond 37 weeks compared with expectant management, though absolute rates are small (0.4 versus 3 deaths per 1000)'. Note that most of the studies included in this review involved induction at 41 weeks, so the findings cannot be applied to induction before this gestation. Research into induction at 39 weeks compared to expectant management has not found a reduction in stillbirth or perinatal mortality [8].

We are very unlikely to get a randomised controlled trial comparing induction with spontaneous labour. However, there is a population-based linked study into short-term and long-term outcomes of induction in uncomplicated pregnancies [9]. This study did not find a reduction in perinatal death (stillbirth and newborn death) with induction of labour.

Risks Associated with Induction

It can be difficult to untangle and isolate the risks involved with induction because usually, more than one risk factor is occurring at once (e.g. syntocinon/pitocin, CTG, epidural). In addition, there are differences in outcomes and risks between women who are having their first baby and women who have previously given birth. Women need to consider their situation and how individual factors influence their risk profile. Care providers should also share individualised information when discussing induction options. Women also need to know that induced labour is not comparable to spontaneous labour. It is very different and requires monitoring and interventions to minimise risks. I cover these differences in depth in a Collective lesson and my book Why Induction Matters. 

Risks associated with the induction procedure and medications

The induction process usually involves some or all of the following (you can read more about the process of induction here). There are a number of minor side effects associated with these medications/procedures (eg. nausea, discomfort etc.) There are also more significant risks:

• Prostaglandins (prostin E2 or cervidil) to ripen the cervix: hyperstimulation resulting in fetal distress and c-section.
• Rupturing the membranes: fetal distress and c-section (see this post).
• IV syntocinon (pitocin): According to the manufacturers, syntocinon administration can result in 'uterine overstimulation which may cause foetal distress, asphyxia and death, or may lead to hypertonicity, tetanic contractions or rupture of the uterus' [10]. Other side effects for the mother include water intoxication, postpartum haemorrhage [11], reduced breastfeeding rates and increased postpartum depression/anxiety [12].

A care provider must gain consent for each intervention and/or medication that they administer, and this requires adequate information about the risks involved (see this post). Medications (syntocinon and prostaglandin) have patient information leaflets in the product packaging. These leaflets should be given to women to read before they consent to receiving medication.

The good news is that because induction is so common, hospital maternity services are usually very good at monitoring and managing complications caused by the induction process. In my experience, bad outcomes from induction occur when there is inadequate monitoring and/or when care providers treat induction like a natural birth. Induction is not a physiological process. If a woman chooses induction, she is opting out of physiological birth.

Risks associated with induction vs spontaneous labour

As I previously mentioned, most research into induction does not compare it with spontaneous labour. In addition, studies often include women with complicated pregnancies, which can skew outcomes. However, a study by Dahlen et al. [9] 'compared intrapartum interventions and outcomes for mothers, neonates and children up to 16 years, for induction of labour (IOL) versus spontaneous labour onset in uncomplicated term pregnancies with live births'. They found that induction of labour reduced the chance of 3rd and 4th-degree tears (likely due to bigger babies in the spontaneous labour group). However, induction increased the chance of perineal repair, i.e. increased overall perineal trauma. Induction for women having their first baby increased the chance of instrumental birth, c-section, epidural, episiotomy, postpartum haemorrhage. For babies, induction increased the chance of neonatal trauma, resuscitation, respiratory disorders and admission to hospital up to 16 years (ear, nose, throat, respiratory problems and sepsis).

Does Induction Reduce the Chance of C-section?

In recent years, care providers have been telling women that induction reduces their chance of having a c-section, and this is supported by general research into induction vs expectant management in mixed-risk populations [7]. This should be no surprise. Modern maternity systems do not support the normal variations of physiological birth. Instead, women are expected to follow non-evidence-based time parameters (see this post). Most women do not fit these timeframes and will end up with interventions, including c-section. However, if a woman is having her labour managed medically, the care provider is better able to make her body fit parameters and avoid a c-section. Hospitals are good at managing the induction of labour because it is their bread and butter.

However, when we move beyond general research, the picture changes. For example, a 2019 review [13] compared the timing of induction for low-risk pregnancies at 41 weeks vs 42 weeks. The review used stricter inclusion criteria than previous reviews to 'enhance the methodological quality and increase the relevance for contemporary maternity care' and reported that: 'Induction at 41+0-6 gestational weeks compared to 42+0-6 gestational weeks was found to be associated with an increased risk of overall cesarean section'.

When it comes to induction and c-section, there are different outcomes for women having their first baby vs women having their 2nd, 3rd, or more baby.

Outcomes for women having their first labour induced 

Inducing a first labour requires higher rates of syntocinon, and the length of labour is usually longer. It is not surprising that first-time mothers are more than 3 x more likely to opt for an epidural during an induction [9], and epidural analgesia increases the chance of ending up with an instrumental birth [14].

The majority of research comparing induction with spontaneous labour in populations of first-time labourers report increased c-section rates [9, 15, 16, 17, 18, 19]. However, one particular US study, the ARRIVE trial [8], is often waved about by care providers in support of the idea that induction reduces the c-section rate for first-time mothers. The ARRIVE trial set out to find out whether inducting first-time mothers at 39 weeks would reduce perinatal death and improve outcomes for babies. It did not, so instead, they decided to report a more newsworthy secondary finding, c-section rates. The study randomly allocated women to either a policy of induction at 39 weeks or a policy of expectant management (including induction after 39 weeks). The women induced at 39 weeks had a lower c-section rate 18.6% vs 22.2%. The study has been constructively critiqued by a number of authors and researchers [20, 21,22]. In summary, the main points are:

• Only 27% of eligible women agreed to participate, suggesting most women did not want to be induced at 39 weeks.
• 94% of the woman were cared for by private obstetricians in US medical settings; therefore, findings cannot be applied to other types of care providers and settings which have much lower rates of c-section and higher rates of spontaneous vaginal birth.
• Care providers were not blinded, and knowing about the trial may have altered their practice and thresholds for intervention.
• Induction reduced c-section rates by 4%. Other factors have a much more significant effect on c-section rates e.g. continuity of midwifery care, place of birth, intermittent auscultation in labour, etc.
• The primary outcome measure for this study was perinatal outcome (i.e. the baby) and induction made no difference to the immediate outcomes for the baby. The study did not address the long-term harm of early birth for the baby (e.g. brain development or breastfeeding).

However, I think the main issue with this study is the application of the findings across other settings. What the study tells us is that a US private obstetrician in a medicalised birth setting is better able to support an induced labour at an earlier gestation than a later induction or a spontaneous labour. When women make decisions about options, they need information that applies to their particular context. For example, a study carried out in Australia [23] found that: 'The likelihood of primary cesarean section following IOL in a Queensland population-based cohort was significantly higher at 38 and 39 weeks, compared with expectant management up to 41⁺⁶ weeks'. This study is more relevant to a woman planning to give birth in Australia. It would be even better if a woman could access information about outcomes in her planned place of birth!

Outcomes for women having their second, third, etc. labour induced

Women who have laboured before respond more effectively to syntocinon (pitocin) because they have more oxytocin receptors in their uterine muscle. Therefore, this group of women are more likely to experience a successful induction and avoid c-section. They are the reason for lower or similar rates of c-section in the mixed populations research. However, again, this is context-specific; for example, the Australian study [23] found an increased rate of c-sections for women being induced with their second, third or more baby. 

The Experience of Induction

The mother's experience of induction

Research into induction tends to focus on physical outcomes rather than women's emotional/psychological experiences of the process.

Choosing induction will alter the birth experience and the options available. Women need to know that agreeing to induction means agreeing to continuous monitoring and an IV drip, which will limit movement. Induced contractions are usually more painful than natural contractions, and the inability to move and/or use warm water (shower or bath) reduces the ability to cope. The result is that an epidural may be needed. An induced birth is not a physiological birth and requires monitoring (vaginal exams) and time frames. For many women, this is fine and worth the risk, but I encounter too many women who are unprepared for the level of intervention required during an induction. This does not mean you have to hand over control of your decisions or your body, and in my book I include a chapter on creating a birth plan for induction of labour.

There have been some studies examining women's experience of induction. A UK study [24] found that: 'Women who were induced were generally less satisfied with aspects of their care and significantly less likely to have a normal delivery. In the qualitative analysis, the main themes that emerged concerned delay, staff shortages, neglect, pain and anxiety in relation to getting the induction started and once it was underway; and in relation to failed induction, the main themes were plans not being followed, wasted effort and pain, and feeling let down and disappointed.'  A German study [25] concluded that: 'Women's expectations and needs regarding IOL are widely unmet in current clinical practice... there is a need for evidence-based information and decisional support for pregnant women who need to decide how to proceed once term is reached.'

There are also problems with researching women's experiences of induction. Firstly, if the woman really believes that her induction was necessary to save her baby, she is likely to be satisfied with her experience. Secondly, a woman cannot compare her induction of labour experience with an experience she did not have ie. physiological labour.

The baby's experience of induction

The baby's experience of an induced labour is different to a spontaneous labour. Firstly, they may not have had time to prepare physically for the demands of labour (lots of changes happen in the last days of pregnancy). Secondly, the pattern and strength of induced contractions are much more likely to cause fetal distress. You can learn more about induction from the baby's perspective in this podcast episode:

Alternatives to Medical Induction

Waiting for spontaneous labour

We know very little about the normal duration of pregnancy because women are often induced before they reach their full gestation. However, when we look at historical research carried out prior to mass induction, 90% of women give birth before 42 weeks [26]. Routine monitoring of the baby while awaiting spontaneous labour does not improve outcomes. An ultrasound scan can only tell you how the baby is at the time of the ultrasound. Post-dates ultrasounds may also identify 'low amniotic fluid' and flag this as a concern when it is normal at the end of pregnancy (see this post). If there are genuine concerns about a baby's well-being, an umbilical artery Doppler assessment should be carried out to assess how well the placenta is functioning. 

'Natural' induction?

There are a number of 'alternative' or 'natural' induction methods available (I have a chapter discussing the evidence for various methods in my book). However, trying to get the body/baby to do something it is not ready to do is still an intervention, whether it is with medicine, herbs, therapies, techniques... or anything else. Interventions of any kind can have unwanted effects and consequences. Medical inductions take place with close monitoring of the mother and baby and access to medical support if a complication arises. Alternative inductions do not have this level of monitoring or backup.

Interventions (massage, acupuncture, etc.) that are aimed at relaxing the mother and fostering trust, patience and acceptance may assist the body/baby to initiate labour if the physiological changes have already taken place.

Summary

The definition of a prolonged pregnancy is 42 weeks or more. However, induction is usually suggested during the 41st week or, more recently, at 39 weeks. Women need to be given adequate information about the risks and benefits involved with either waiting or inducing to make the choice that is right for them. There is no risk-free option.

Further Resources

You can find more information on this topic in my books and Reclaiming Childbirth Collective lessons.

References

1. AIHW (2024) Australia's mothers and babies
2. Morken et al. (2011) Recurrence of prolonged and post-term gestational age across generations
3. McAlpine et al. (2016) The association between third trimester multivitamin/mineral supplements and gestational length in uncomplicated pregnancies
4. Carroll et al. (2020) A review of the evidence for placental ageing in prolonged pregnancy
5. Qi et al. (2022) Aging of the placenta
6. Muglu et al. (2019) Risk of stillbirth and neonatal death with advancing gestation at term
7. Middleton et al. (2020) Induction of labour at or beyond 37 weeks' gestation
8. Grobman et al. (2018) Labor induction vs. expectant management of low-risk pregnancy
9. Dahlen et al. (2021) Intrapartum interventions and outcomes for women and children following induction of labour at term in uncomplicated pregnancies
10. Syntocinon medication information leaflet
11. Grotegut et al. (2011) Oxytocin exposure during labor among women with postpartum haemorrhage secondary to uterine atony
12. Gu et al. (2016) Intrapartum synthetic oxytocin and its effects on maternal well-being at 2 months postpartum
13. Rydahl et al. (2019) Effects of induction of labor prior to post-term in low-risk pregnancies
14. Anim-Somuah (2018) Epidurals for pain relief in labour
15. Kjerulff et al. (2017) Labor induction and cesarean delivery
16. Selo-Ojeme (2011) Is induced labour in the nullipara associated with more maternal and perinatal mortality?
17. Davey & King (2016) Caesarean section following induction of labour in uncomplicated first-births
18. Butler et al. (2024) Induction of labor and caesarean birth in lower-risk nulliparous women at term
19. Ehrenthal (2010) Labor induction and the risk of cesarean delivery among nulliparous women at term
20. Carmichael & Snowden (2019) The ARRIVE Trial: Interpretation from an epidemiological perspective
21. Davies-Tuck et al. (2018) Why ARRIVE should not thrive in Australia
22. Scialli (2019) Induction of labor at term
23. Hu et al. (2024) Likelihood of primary caesarean section following induction of labor in singleton cephalic pregnancies at term, compared with expectant management
24. Henderson & Redshaw (2013) Women's experiences of induction of labor
25. Schwartz et al. (2016) Women's perceptions of induction of labour outcomes
26. Bergsjø et al. (1990) Duration of human singleton pregnancy