Vol. 22 No 3 | Spring 2020
Managing pregnancies complicated by obesity
A/Prof Glyn Teale

In Australia, approximately 45% of women giving birth are overweight or obese, ranging from 42% in New South Wales to 52% in South Australia and Tasmania. (Table 1). Some rural areas report rates over 65%.1

Table 1. Prevalence of overweight and obesity across Australia 2017.

BMI (kg/m2) NSW Vic Qld WA SA Tas ACT NT Australia
Overweight: 25.0–29.9 24.3 26.9 23.4 28.2 27.7 26.7 26.7 28.3 25.6
Class I and II obesity: 30.0–39.9 15.3 17.1 17.4 17.8 20.8 21.3 16.9 20.6  17.0
Class III obesity: 40.0–49.9 2.4 2.7 3.1 2.3 3.6 4.2 3.2 2.3 2.7
Extreme obesity: 50 and over 0.2 0.3 0.3 0.2 0.5 0.6 0.5 0.2 0.3
Total overweight & obese 42.2   47.0  44.2 48.5 52.6  52.8  47.3   51.4 45.6

Data derived from: www.aihw.gov.au/reports/mothers-babies/australias-mothers-and-babies-2017-in-brief/data

Almost all adverse outcomes of pregnancy are over-represented in overweight or obese women.2

Even in the absence of complications such as diabetes or hypertension, obese pregnant women still face a significantly increased risk of stillbirth; a risk that is discernible at a body mass index (BMI) of ‘only’ 30, so it is not just the extremes of BMI that are concerning (Table 2).3 The sheer prevalence of class I and II obesity (around 17%) seems to obtund the level of clinician concern as there is a tendency to see this as ‘normal’, potentially resulting in missed opportunities to mitigate risk. Reports that outcomes for women with ‘diabesity’ – obesity and gestational diabetes mellitus (GDM) – are better than those for obesity alone suggest that the risks of obesity are not recognised and managed the way they are for women with GDM and obesity who receive intensive surveillance.4

There are several peak-body-issued guidelines for the management of obese pregnant women. There are disparities in these guidelines and some provide limited advice on key aspects of care.5 It is notable that almost all of the recommendations in the RANZCOG ‘Management of Obesity in Pregnancy’ statement are graded as ‘consensus based’.6 This highlights a paucity of high-level evidence for mitigating the risks, and much of the advice contained within the statement is common sense. Rather than repeating recommendations, the following discussions will concentrate on practical aspects of care, with an update on recent evidence where available.

Table 2. Adjusted hazard ratio for stillbirth and class of obesity.

Obesity class Adjusted hazard ratio  95% confidence intervals
Class I
(BMI 30–34.9)
1.71 1.62–1.83
Class II
(BMI 35–39.9)
2.04 1.89–2.21
Class III
(BMI ≥40)
2.50 2.28–2.74
Extreme BMI (≥50) 3.11 2.54–3.81

Hazard ratio calculated with Cox proportional hazard regression analysis; baseline comparison group: normal weight (BMI 20–24.9). Derived from Yao et al, AJOG, 2014.7

Early pregnancy

Counselling about increased risks related to weight is important and may assist compliance with the various additional surveillance and screening measures that will be offered. Many clinicians find conversations about weight difficult to initiate and there is a balance to be achieved between providing information of risk and potential negative patient experience.8 A factual, non-judgemental reference to being ‘above a healthy weight’ avoids pejorative language and seems to be acceptable.

‘I see from your weight that you are currently above the healthy weight range. I would suggest a few extra investigations and monitoring through your pregnancy that I would like to talk to you about.’

Obesity is associated with a greater likelihood of major depressive symptoms antenatally and postnatally, supporting the need for routine assessment for depression.9

Conversations about exercise, diet and weight gain limitation should be routine. Most pregnant women, of any weight, can safely undertake a daily 30–45-minute brisk walk or regular swimming; RANZCOG patient information leaflets can support the advice.

‘Regular, daily exercise can be helpful for you and your baby. It can also help limit how much weight you gain. Do you think you would be able to fit in a 30-minute walk every day in your pregnancy?’

Information on diet adjustments and weight gain limitation can be supported by referral to a dietician where available or with online resources (such as www.eatforhealth.gov.au). While it has proven difficult to limit weight gain in many RCTs, meta-analysis supports the need to at least attempt to adhere to weight gain advice with subsequent potential reduction in a variety of pregnancy complications.10

When ordering non-invasive prenatal testing (NIPT) it is important to counsel on the significantly increased risks of not obtaining a result due to low fetal fraction (no-call); approximately 5% in class I obesity rising to 10% in Class II, and even higher in Class III in some studies.11 The most appropriate response to this situation remains unclear, but the possibility should be discussed beforehand.

Potential mitigation of the increased risk of pre-eclampsia supports routine administration of low-dose aspirin and calcium supplementation for obese women. With an understandable resistance to taking medication in pregnancy, compliance with this recommendation is low and may be improved by the provision of written information.

Women who have Class II or III obesity have a 20% chance of having an abnormal early (<20 weeks) oral glucose tolerance test.12 This screen is widely practiced, although there is little evidence that routine early screening for GDM is beneficial to outcomes. For those who screen negative, this should be repeated at 26–28 weeks.

Routine ultrasound scanning at approximately 20 weeks is less likely to identify significant abnormalities in obese women and it is worth warning of the frequent need to repeat the investigation which can be time consuming and anxiety provoking. It may be helpful to consider specialist referral to a COGU or MFM subspecialist for women with Class III or extreme BMI.


Given the substantial risk of complications, a low-risk schedule of appointments is not appropriate for overweight or obese women. The frequency of visits should be determined by the presence of co-morbidities and the BMI.

Regular urine testing for proteinuria is useful given the increased risk of UTIs and asymptomatic bacteriuria and to support possible early detection of pre-eclampsia.

Routine 26–28 week bloods should assess for adequate haemoglobin and, where necessary, iron stores to mitigate the increased risk of a postpartum haemorrhage.

Undetected growth restriction is likely the underlying cause of the increased stillbirth risk and clinical assessment of fetal growth with symphysio-fundal height is unreliable. For women with Class III obesity, without another indication for growth ultrasounds, serial scans at 28, 33 and 36 weeks are reasonable. RANZCOG recommends offering a routine third trimester scan.13 Unfortunately, routine ultrasound scans are also subject to significant error and a one-off third trimester scan, especially if performed at 35–36 weeks, is a poor predictor of fetal size.14 Data from Western Health (accepted for publication) suggest that the sensitivity for detecting growth restriction in women with Class II/III obesity is below 10% and only approximately 50% of women whose scan suggests macrosomia will go on to deliver a ‘large for dates’ baby. The poor prediction of macrosomia is important to consider in counselling for mode of delivery, and the problems of identifying growth restriction should be remembered when deciding on timing of delivery.

The value of routine fetal movement monitoring in the general obstetric population remains uncertain; however, as already highlighted, obesity is significantly associated with a higher risk of stillbirth. A recent systematic review of the significance of decreased fetal movements (DFM) suggests that the perception of fetal movements is not affected by maternal body size, although more research is needed for women with Class II and III obesity.15 Presentations for DFM are more common and, importantly, may be more strongly associated with adverse outcomes in women with obesity. Thorough assessment of DFM is appropriate and the management should consider the increased risk of stillbirth and the challenges of assessing fetal wellbeing.

Timing and mode of delivery

The risks of stillbirth rise exponentially at 39 weeks for women with extreme obesity (BMI>50);16 this supports routinely offering induction or delivery at 38–39 weeks for this group. There is increasing evidence that routine induction of obese women is not associated with an increased risk of caesarean birth and may be associated with reduced maternal and neonatal morbidity.17 18 Given the lack of ability to reliably monitor fetal growth in women with Class III obesity despite regular ultrasounds, Western Health also routinely offers induction at 39 weeks for this group. The appropriate management for women with Class I and II obesity is unclear and being actively debated.

The likelihood of emergency caesarean is strongly influenced by BMI.19 Over 40% of spontaneously labouring nullipara with Class III obesity will birth by emergency caesarean. The reasons for this increase are unclear but repeatedly reported and this should be discussed with the woman. For some women, the balance of risks may favour elective caesarean. The counselling will be influenced by comorbidities and history, but it is important that the information is exchanged. Failing to inform of the options and risks increases the chance of dissatisfaction and complaint.

For those who wish to contemplate vaginal birth after caesarean (VBAC), obesity negatively impacts the chance of successful vaginal delivery and the risks of serious maternal and neonatal complications rise.20 Chances of success may be as low as 20% for women with Class III obesity, so it is vital to provide accurate information when counselling about risks and benefits; using success figures of circa 75%, as might be provided to women in the healthy weight range, is unrealistic for obese women.

Intrapartum care

Time to achieve full dilatation is prolonged for obese women, whether they labour spontaneously or following induction.21 22 Most of this extra time is in the latent phase, with active first stage and second stage being similar. There is also a greater chance of failed induction, which may be as high as 37.5% for Class III obesity, with Bishop score reliably indicating success rates.23 Higher doses and longer exposure to oxytocin is also needed to achieve vaginal delivery.24 Overall, additional patience is probably helpful to allow active labour to establish.

Staff caring for obese labouring women should be appropriately skilled and experienced to mitigate the various complications that are more likely to occur. With increased likelihood of an undetected growth restricted baby, optimal fetal monitoring is important but harder to achieve and there is commonly a need to resort to internal monitoring.25 The substantially increased risks of a postpartum haemorrhage should be mitigated by active management of the third stage, ensuring adequate large bore cannulation, and early recourse to additional measures.

Considerations for the care of obese women who need to go to theatre intra- or postpartum were reviewed in the Spring 2019 issue of O&G Magazine.26

Postpartum care

Women in higher obesity classes are progressively less likely to initiate breastfeeding.27 Women with the highest BMIs should be particularly counselled on the benefits of breastfeeding and extra support should be provided.

Routine postpartum thromboprophylaxis is recommended for many obese women – for more information see www.bettersafercare.vic.gov.au. Compliance studies suggest that more than 30% fail to comply with frequency or length of treatment recommendations;28 specific counselling for obese women is important to ensure they understand the risks of non-compliance.


Much of the advice for the management of women who are obese and pregnant is common sense. Given the increased likelihood of many complications, it is important to provide appropriate information in a sensitive but accurate manner. The tendency to normalise obesity, particularly in the absence of associated complications, can lead to a failure to mitigate risks; stillbirth risk is considerable and management decisions should reflect this risk at all levels of obesity. Hopefully, by having the increased risks of obesity ‘front of mind’ we can reduce the adverse outcomes that are so much more prominent.


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