Managing Post-Term Pregnancy
Published: 03 June 2020
Published: 03 June 2020
The timing of birth has a significant impact on neonatal wellbeing. Too early or too late can be the difference between life and death, yet by intervening in the natural timing of birth, existing problems can be exacerbated, or entirely new problems created.
Post-term pregnancy is a pregnancy that extends to 42 weeks of gestation or beyond (Galal et al. 2012).
However, interchangeable use of terms such as ‘post-term’, ‘post-dates’ and ‘prolonged’ pregnancy can cause confusion and misunderstanding. A more precise and internationally recognised definition is offered by the World Health Organization (WHO) and the International Federation of Gynaecology and Obstetrics (FIGO), who state that:
(Association of Ontario Midwives 2010)
In recent years the management of post-term pregnancy has been challenged. This is mainly due to emerging evidence that the risk of complications associated with post-term pregnancy starts to increase before 42 weeks of gestation.
For example, the incidence of stillbirth increases from 39 weeks onwards, with a sharp rise after 40 weeks of gestation (Galal et al. 2012). This means that accurate pregnancy dating is crucial to the diagnosis and management of post-term pregnancy.
The use of standard clinical criteria is the most common method of calculating the estimated date of delivery (EDD). For example:
However, inaccuracies may occur using clinical criteria alone due to cycle irregularity, recent use of hormonal contraception, or early bleeding in pregnancy. The result is that gestational age can easily be overestimated, increasing the risk of the pregnancy being labelled post-term.
This is why ultrasound scanning has become so valuable in pregnancy dating, reducing the overall rate of post-term pregnancy from 10-15% down to approximately 2-5% (Galal et al. 2012).
Hart (2004) suggests that foetal postmaturity syndrome is a continuum, becoming more likely the longer the pregnancy progresses past the due date. They also report on findings that suggest careful monitoring whilst waiting for labour to begin results in fewer caesarean sections without any rise in the stillbirth rate.
The key points here are that the risks of waiting for the onset of spontaneous labour should be compared with the risks of interventions such as induction. In other words, a post-dates pregnancy can be safely monitored until labour begins, or until there are sound clinical indications for induction (Hart 2004).
This ‘wait and watch’ approach is also endorsed by Caughey and Bishop (2006), who suggest that the counselling of women who progress past their EDD should include comparing the risks of induction to that of expectant management.
Hart (2004) notes that post-dates alone are not associated with poor pregnancy outcomes; it is extreme post-dates or those associated with poor fetal growth or developmental abnormalities that result in an increased risk of stillbirth.
However, if growth restriction and birth defects are removed, there is no statistical increase in risk until a pregnancy reaches 42 weeks, and no significant risk until after 43 weeks.
Factors that are known to contribute to prolonged pregnancy include:
(Association of Ontario Midwives 2010)
Population characteristics that affect the prevalence of postmaturity include:
(Galal et al. 2012)
Although the link between body mass index and the duration of pregnancy is not clearly understood, it seems that women who are obese have a higher incidence of post-term pregnancy (Galal et al. 2012).
Halloran et al. (2011) also note that post-term delivery is becoming more common with increasing pre-pregnancy weight and increasing maternal weight gain. Early antenatal advice on maintaining a healthy weight could help reduce the risk of post-term pregnancy.
A key danger of postmaturity is the increased risk of fetal and neonatal complications. The perinatal mortality rate at 42 weeks of gestation is twice as high than it would be at term. Furthermore, at 43 weeks the risk is four times greater, and at 44 weeks the risk is five to seven times greater (Galal et al. 2012).
About 20% of post-term foetuses are thought to have dysmaturity syndrome, where infants have characteristics resembling chronic intrauterine growth restriction from utero-placental insufficiency. These characteristics includes:
Post-term pregnancies are also at increased risk of umbilical cord compression from oligohydramnios, meconium aspiration and short-term neonatal complications such as hypoglycaemia, seizures, and respiratory insufficiency (Galal et al. 2012).
To prevent a pregnancy from becoming post-term, induction of labour is often recommended. Yet, there is no conclusive evidence that this intervention improves foetal, maternal and neonatal outcomes more than expectant management.
After careful identification and the exclusion of specific risks, it would seem appropriate to let women make an informed decision about which management pathway they wish to follow (Mandruzzato et al. 2010).
Various methods have been suggested to initiate spontaneous labour and avoid the need for a post-term induction, including:
These interventions are used because they are believed to support the natural changes at the end of pregnancy, rather than have a specific ability to initiate labour, though not all are supported by high-quality evidence-based research.
Kortekaas et al. (2019) note that the timing of induction for post-term pregnancies can also differ between midwifery-led and obstetrician-led care, with women in midwifery-led care receiving more membrane sweeping compared to women receiving obstetrician-led care.
Yet, as long as specific risks for adverse outcomes are identified, Hart (2004) asserts there is no evidence to support routine induction of labour in a post-date pregnancy.
For many midwives, the pressure to opt for labour induction also goes against the natural tendency to view a post-date pregnancy as a normal variation. Whilst there is often great pressure to accept intervention, there is no conclusive evidence that prolongation of pregnancy is a major risk factor in and of itself.