Prolonged pregnancy

Prolonged pregnancy requires careful monitoring and management to reduce the risk of adverse consequences for mother and baby.

62.1 Background

For the purposes of these Guidelines, ‘term’ is defined as 37⁰ to 41⁶ weeks gestation and ‘post-term’ or ‘prolonged pregnancy’ as ≥42⁰ weeks AIHW 2018a. 

However, pregnancy length may differ depending on the woman’s ethnicity, which has implications for monitoring in late pregnancy. A Victorian study found that the average natural onset of labour occurred at 39 weeks in women born in South Asian countries compared to 40 weeks in women born in Australia and New Zealand Davies-Tuck et al 2017. 

Identification of prolonged pregnancy relies on accurate estimation of date of birth, which is discussed in Chapter 20 of the Guidelines. The agreed due date should not be changed without the advice from another health professional with considerable experience in antenatal care.

62.1.1 Incidence of prolonged pregnancy

In 2016, 91.7% of Australian babies were born at 37⁰ to 41⁶ weeks and 0.6% were born post-term AIHW 2018a:

• for term births, labour was spontaneous in 48.8% of women and induced in 31.2% of women and 20% of women had no labour
• for post-term births, labour was spontaneous in 35.02% of women and induced in 59.8% of women and 5.1% of women had no labour.

While the aetiology of post-term birth is not well elucidated Mandruzzato et al 2010, risk factors such as obesity, nulliparity and maternal age greater than 30 years have been associated with an increased risk of post-term birth (Arrowsmith et al 2011; Caughey et al 2009; Heslehurst et al 2017; Roos et al 2010). Placental senescence may play a role in the pathophysiology of post-term birth (Mandruzzato et al 2010), and genetic/epigenetic factors have also been implicated (Schierding et al 2014).

62.1.2    Risks associated with prolonged pregnancy

In a study from the Norwegian Birth Registry Heimstad et al 2008, the perinatal death rate was 0.018% at day 287 (41 weeks) and 0.51% at day 302+ (>43 weeks). These findings are important in that, even in a setting where early booking allows accurate assessment of gestational age and antenatal services are accessible for most women, post-term pregnancy constitutes a high-risk situation, especially for the baby. In another Norwegian study of nearly two million births from 1967 to 2006, the risk of post-term infant death was strongly associated with fetal growth restriction (OR 3.1; 95%CI 2.5 to 4.0 for non-small-for-gestational-age post-term infants vs OR 4.9, 95%CI 3.8 to 6.4 in small-for-gestational-age post-term infants) Morken et al 2014.

Among babies born post-term in Australia in 2016 AIHW 2018b:

• 52% were born vaginally, 31% were born by caesarean section and 17% were instrumental vaginal births (among babies born at 37-41 weeks, 54% were born vaginally, 33% by caesarean section and 13% were instrumental vaginal births)
• 5.9% weighed more than 4,500 g (compared with 1.3% of babies born at 37 to 41 weeks)
• 1.9% had Apgar scores lower than seven at 5 minutes (compared with 1.4% of babies born at 37 to 41 weeks).

The perinatal death rate was 2.2 per 1,000 births (compared to 1.5 per 1,000 births for babies born at 37 to 41 weeks) AIHW 2018b. 

Potential risks for the mother associated with post-term pregnancy include prolonged labour, postpartum haemorrhage and perineal tears. It is likely that some of these outcomes result from intervening when the uterus and cervix are not ready for labour Caughey & Musci 2004. Women may also experience anxiety, particularly if the woman perceives her prolonged pregnancy as high risk ACOG 2004; Heimstad et al 2007.

62.2 Options in prolonged pregnancy

Policies vary on intervening in low-risk prolonged pregnancies. Offering labour induction after 41 weeks is recommended in the United Kingdom NICE 2008; updated 2017 and the United States ACOG 2014. Factors to be considered include the results of fetal assessment, the woman’s Bishop’s score, gestational age and the woman’s preferences, after discussion of available alternatives and their risks and benefits ACOG 2004; Norwitz et al 2007.

62.2.1 Sweeping the membranes

Procedures for cervical ripening, such as membrane sweeping, may be of benefit in preventing prolonged pregnancy, particularly in first pregnancies Mandruzzato et al 2010. Membrane sweeping involves the health professional introducing a finger into the woman’s cervical os (the opening of the cervix into the vagina) and ‘sweeping’ it around the circumference of the cervix during an vaginal examination, with the aim of separating the fetal membranes from the cervix and triggering the release of prostaglandins NICE 2008; updated 2017.

A Cochrane review (n=2,797) Boulvain et al 2005 found an association between membrane sweeping, and reduced frequency of pregnancy continuing beyond 41 weeks (RR: 0.59; 95%CI: 0.46 to 0.74) and 42 weeks (RR: 0.28; 95%CI: 0.15 to 0.50). The strength of the review was limited by small sample sizes and heterogeneity of the studies and possible publication bias for some outcomes. 

A more recent systematic review found that membrane sweeping is a safe, effective and inexpensive method of labour induction Heilman & Sushereba 2015. Membrane sweeping can be performed in Group B Streptococcus-positive women with studies showing no increase in adverse outcomes Heilman & Sushereba 2015. However, there are no data concerning women with HIV or hepatitis infection.

62.2.2 Induction of labour to prevent prolonged pregnancy

Induction of labour for prolonged pregnancy is widely practised with the aim of preventing stillbirth and reducing perinatal morbidity eg shoulder dystocia in large babies.

Among Australian women (of all gestations) whose labour was induced in 2016, 60.7% had a vaginal birth, 18.6% had an assisted vaginal birth (forceps or vacuum extraction) and 20.8% had a caesarean section AIHW 2018b.

A recent Cochrane review assessed the evidence on induction of labour for improving birth outcomes for women at or beyond term Middleton et al 2018. The timing of induction in the trials in the Cochrane review varied and, in some trials, women were induced before pregnancy was prolonged. 

For the purposes of these Guidelines, subgroup analysis was conducted for inductions at 41⁰ to 41⁶, at 42⁰ to 42⁶ weeks and mixed timing crossing 41 and 42 weeks. Trials in which induction occurred before 41 weeks were not included. An RCT published subsequent to the Cochrane review (Grobman et al 2018) was not included in the analysis as women were induced at 39⁰ to 39⁴ weeks (ie before pregnancy was prolonged).

There were no clear differences between subgroups for any outcome. Compared with expectant management, labour induction at 41⁰ to 42⁶ weeks was associated with: 

• fewer (all-cause) perinatal deaths (RR 0.37, 95% CI 0.10 to 0.87; 15 RCTs, n=8,408, moderate certainty) 
• fewer babies with an Apgar score lower than seven at 5 minutes (RR 0.67, 95% CI 0.46 to 0.98; 12 RCTs, n=7,913, low certainty)
• fewer babies with macrosomia (RR 0.65, 95% CI 0.53 to 0.79, 8 RCTs, n=4,736, low certainty), with a higher reduction among women induced at 41⁰ to 41⁶ weeks (RR 0.50, 95% CI 0.37 to 0.66, 6 RCTs, n=1,225)
• fewer caesarean sections (RR 0.90, 95%CI 0.83 to 0.98; 17 RCTs, n=8,803, moderate certainty). 

There were no clear differences between expectant management and induction groups for stillbirth (RR 0.34, 95% CI 0.09 to 1.24, 15 RCTs, n=8,404, low certainty), neonatal death (RR 0.37, 95% CI 0.10 to 1.38, 15 RCTs, n=8,408, low certainty) or rates of admission to neonatal intensive care (RR 0.88, 95% CI 0.76 to 1.01, 9 RCTs, n=7,397, low certainty).

The Cochrane review concluded that the optimal timing of offering induction of labour to women at or beyond term warrants further investigation, as does the risk profiles of women and their values and preferences Middleton et al 2018.

62.3 Monitoring of women with prolonged pregnancy

Increased fetal and maternal monitoring aims to identify risk of adverse outcomes and ensure timely induction of labour if indicated (eg fetal compromise or oligohydramnios). Definitive recommendations for fetal monitoring are hampered by the absence of RCTs demonstrating that fetal monitoring decreases perinatal morbidity or mortality between 41 and 42 weeks or in post-term pregnancies ACOG 2014. 

After 42 weeks, increased antenatal monitoring may include twice-weekly cardiotocography and ultrasound estimation of maximum amniotic pool depth NICE 2008; updated 2017 although the evidence to support this practice is not strong. Specialist referral or consultation is likely to be required.

62.4 Discussing prolonged pregnancy

When a woman is approaching prolonged pregnancy, discuss her preferences and provide information on the risks and benefits of different management options.

This may include that:

• most women go into labour spontaneously by 42 weeks
• there are risks associated with pregnancies that last longer than 42 weeks
• women with prolonged low-risk pregnancies may be offered membrane sweeping to ‘trigger’ labour, which involves the health professional separating the membranes from the cervix as part of a vaginal examination; it is safe but may cause discomfort and vaginal bleeding
• if pregnancy is prolonged, additional monitoring and management plans will be offered following specialist consultation, to reduce the risk of adverse outcomes 
• the importance of contacting a health professional promptly if they have any concerns about changes in fetal movements (including decreases, absence or unusually increased) (see Chapter 22 of the Guidelines).

62.5 Practice summary: prolonged pregnancy

When

At antenatal visits from 39 weeks onwards.

Who

• Midwife
• GP
• obstetrician
• Aboriginal and Torres Strait Islander Health Practitioner
• Aboriginal and Torres Strait Islander Health Worker
• multicultural health worker.

What

• Discuss the likelihood of prolonged pregnancy
  Explain to the woman that pregnancy beyond 42 weeks is unlikely if dating is accurate.
• Discuss why interventions may be offered
  Explain that the risk of complications increases from 42 weeks pregnancy. Decisions about management are made after considering the risks and benefits and taking the woman’s preferences into account.
• Discuss the need for fetal surveillance
  Explain that increased fetal monitoring is necessary from 41 weeks, to ensure that there are no risks to the baby from the pregnancy continuing.
• Take a holistic approach
  As well as the potential for women to experience anxiety if pregnancy is prolonged, consider practical difficulties (eg when the woman has travelled to give birth or arranged additional support around the estimated date of birth) and provide advice on relevant community supports (eg available financial assistance).

62.6 Resources

• ACOG (2014) Management of Late-Term and Postterm Pregnancies, ACOG Practice Bulletin Number 146: Obstet Gynecol. 2014; 124:390-396.
• Gardener G, Daly L, Bowring V et al (2017) Clinical practice guideline for the care of women with decreased fetal movements. Brisbane: The Centre of Research Excellence in Stillbirth.