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Pregnancy Care Guidelines

19 Weight and body mass index

Body mass index (prior to pregnancy or at the first antenatal visit) and weight gain during pregnancy are among the important determinants of the health of both mother and baby.

19.1 Background

The worldwide prevalence of obesity has risen dramatically in the past few decades and Australia is among those countries with the highest prevalence. There is a well-documented increased risk of complications for women who are overweight or obese during pregnancy. Conversely, being underweight during pregnancy can also affect the baby’s health.

19.1.1 Calculating and interpreting BMI 

Body mass index (BMI) is an index of weight-for-height that is commonly used to classify underweight, overweight and obesity in adults. It is calculated by dividing weight by the square of height: weight (kg)/height (m)2. The WHO classification of BMI classification is given in Table D2.

Table D1: Classification of adult underweight, overweight and obesity according to BMI

A list of BMIs (noted in kilograms by square metres) and classifications.
BMI (kg/m2) Classification
<18.50 Underweight
18.5–24.9 Healthy weight 
25.0–29.9 Overweight
≥30.0 Obesity

Source: (WHO 2000).

19.1.2 Weight classification during pregnancy in Australia

Among women who gave birth in Australia in 2013 (AIHW 2016):[16]

  • 19% were obese, 24% were overweight, 46% were in the normal weight range and 3% were underweight at the beginning of their pregnancy
  • Aboriginal and Torres Strait Islander women were more likely than non-Indigenous women to be obese (25%) or underweight (7%), less likely to be in the normal weight range (37%) and had a similar likelihood of being overweight (23%)
  • compared to women born in Australia, women born overseas were less likely to be obese (13 vs 21%) and more likely to be in the healthy weight range (51 vs 44%), rates of overweight (23 vs 25%) or underweight (4 vs 3%) were similar
  • compared to women in the highest socioeconomic status quintile, those in the lowest quintile were more likely to be obese (25 vs 12%), less likely to be in the healthy weight range (40 vs 53%) and had a similar likelihood of being overweight (25 vs 23%) or underweight (both 4%)
  • obesity was most common in very remote areas (25 vs 17% in major cities), prevalence of overweight was similar across geographical regions, prevalence of healthy weight decreased with increasing remoteness (47% in major cities to 36% in very remote areas) and underweight was more common in very remote areas (6 vs 3% in major cities).

19.1.3 Risks associated with a low or high BMI

Underweight

A low BMI is associated with increased risk of:

  • preterm birth
  • small-for-gestational-age babies
  • low birth weight (Liu et al 2016).

A BMI <20 has been associated with an increased risk of:

  • a low birth weight baby among Aboriginal and Torres Strait Islander women (Panaretto et al 2006).

Overweight

BMI >25 has been linked with:

  • stillbirth (Chu et al 2007a)
  • congenital anomalies (Chu et al 2007b; Oddy et al 2009; Stothard et al 2009)
  • neural tube defects (Rassmussen et al 2008; Oddy et al 2009; Stothard et al 2009)
  • preterm birth (Viswanathan et al 2008; McDonald et al 2010)
  • low birth weight (Viswanathan et al 2008; McDonald et al 2010)
  • large-for-gestational-age babies (HAPO 2010)
  • gestational hypertension (Callaway et al 2006;  HAPO 2010)
  • pre-eclampsia (HAPO 2008)
  • gestational diabetes (Chu et al 2007b; Callaway et al 2006)
  • postpartum haemorrhage (CMACE & RCOG 2010)
  • major depressive disorders (Bodnar et al 2009).

Obesity

BMI ≥30 is also linked to the above outcomes and to:

  • an inability to initiate breastfeeding (Viswanathan et al 2008)
  • postpartum weight retention (Thornton et al 2009)
  • increased rate of caesarean section (Callaway et al 2006; Chu et al 2007c; HAPO 2010).

19.1.4 Weight gain during pregnancy

While BMI prior to or early in pregnancy is independently associated with pregnancy outcomes, the amount of weight gained during pregnancy is also a contributing factor (Nohr et al 2008; Viswanathan et al 2008). The US Institute of Medicine (IOM) provides guidance on weight gain in pregnancy based on pre-pregnancy BMI (see Table D3).

Table D2: IOM 2009 recommendations for weight gain in pregnancy

A list of IOM 2009 recommendations. It contains 3 columns: Pre-pregnancy BMI (kg/m2), weight gain (kg) and Rates of weight gain 2nd and 3rd trimester (kg/wk).
Pre-pregnancy BMI (kg/m2) Recommended weight gain (kg) Rates of weight gain 2nd and 3rd trimester (kg/wk)
<18.5 12.5–18.0 0.51 (0.44–0.58)
18.5 to 24.9 11.5–16.0 0.42 (0.35–0.50)
25.0 to 29.9 7.0–11.5 0.28 (0.23–0.33)
≥30 5.0–9.0 0.22 (0.17–0.27)

Note: The IOM recommendations are consensus-based and were written in 2009. Since that time, evidence has accumulated on the need to tailor weight gain recommendations to the individual in women with a BMI ≥40; further research is required into the applicability of the IOM recommendations in these women. It should also be noted that the recommended weight gain ranges are indicative only and provide suggested limits rather than specific goals.

Source: (NHMRC 2013) based on (IOM 2009).

A recent systematic review of 23 studies from the US, Europe and Asia found 47% of women gained more and 23% gained less weight during pregnancy than recommended (Goldstein et al 2017). In an Australian study (de Jersey et al 2012), 36% of women gained weight according to guidelines, 26% gained inadequate weight and 38% gained excess weight. Among overweight women, 56% gained weight in excess of the IOM guidelines compared with 30% of those who started with a healthy weight (P < 0.001).

19.1.5 Risks associated with low or high weight gain during pregnancy

A recent systematic review found (Goldstein et al 2017):

  • gestational weight gain below the recommendations was associated with higher risk of small-for-gestational age (OR 1.53, 95% CI 1.44 to 1.64) and preterm birth (OR 1.70, 95% CI 1.32 to 2.20), there was no clear difference in risk of caesarean section (OR 0.98, 95% CI 0.96 to 1.02).
  • gestational weight gain above the recommendations was associated with higher risk of large-for-gestational age (OR 1.85, 95% CI 1.76 to 1.95), macrosomia (OR 1.95, 95% CI 1.79 to 2.11) and caesarean section (OR 1.30, 95% CI 1.25 to 1.35).

Gestational weight gain above recommendations is also associated with hypertension (Crane et al 2009) and pre-eclampsia (DeVader et al 2007). High gestational weight gain in women who are obese has been associated with neonatal metabolic abnormalities (Crane et al 2009).

Weight gain before and during pregnancy not only affects the current pregnancy but may also contribute to future weight retention (Nohr et al 2008; Viswanathan et al 2008; Siega-Riz et al 2009).

19.2 Assessing BMI and weight gain

Routinely measuring women’s height and weight and calculating BMI at an early antenatal contact is recommended in New Zealand (NZ MoH 2014), the United Kingdom (NICE updated 2016), the United States (ACOG 2013) and in Australia (RANZCOG 2017).

Encouraging self-monitoring of weight is recommended in New Zealand (NZ MoH 2014), while the NICE guidelines recommend confining repeated weighing to circumstances in which clinical management is likely to be influenced (NICE updated 2016). In Canada, weight gain tracking charts have been developed for the different weight classifications (Health Canada 2010).

Guidelines on the management of obesity in pregnancy have been developed in Australia (RANZCOG 2017), the United Kingdom (CMACE & RCOG 2010) and Canada (SOGC 2010). These guidelines are consistent in recommending that women who are obese be advised of the risks associated with obesity in pregnancy.

19.2.1 Measuring height and weight and calculating BMI

Routine measurement of women’s weight and height and calculation of BMI at the first antenatal contact allows identification of women who require additional care during pregnancy. When there is an accurate record of a woman’s pre-pregnancy BMI, this may be used to inform gestational weight gain. Note that the BMI can be less accurate for assessing healthy weight in certain groups due to variations in muscle mass and fat mass (eg cut-offs lower than the WHO classifications are recommended for women from Asian backgrounds and higher cut-offs are recommended for women from Pacific Islands) (Duerenberg et al 2002; James et al 2004; Depres & Tchernof 2007).

Recommendation

  • Consensus-based
  • VIII

Measure women’s weight and height at the first antenatal visit and calculate their body mass index (BMI) to inform gestational weight gain.

Approved by NHMRC in December 2011; expires December 2016 UNDER REVIEW

Recommendation

  • Consensus-based
  • IX

Give women advice about appropriate weight gain during pregnancy in relation to their pre-pregnancy BMI (if recorded) or their BMI at the first antenatal visit.

Approved by NHMRC in December 2011; expires December 2016 UNDER REVIEW

19.2.2 Discussing weight and weight gain with women

Women who have a BMI that is below or above the healthy range are likely to require additional care during pregnancy. For women with an elevated BMI, there may be additional implications for care during pregnancy (eg the potential for poor ultrasound visualisation) and the birth (eg need for the birth to take place in a larger centre, difficulties with fetal monitoring). Relevant risks associated with a woman’s BMI should be explained and the woman given the opportunity to discuss these and how they might be minimised.

Recommendation

  • Practice point
  • X

Adopting a respectful, positive and supportive approach and providing information about healthy eating and physical activity in an appropriate format may assist discussion of weight management. This should be informed by appropriate education for health professionals.

Approved by NHMRC in December 2011; expires December 2016 UNDER REVIEW

19.2.3 Recent evidence on routine weight monitoring

No systematic reviews on weight monitoring were identified. A recent Australian RCT (n=782) (Brownfoot et al 2015; Brownfoot et al 2016) addressed regular weighing at antenatal care visits plus advice on weight gain versus usual care. The study found no clear difference in weight gain, proportion of women gaining more weight than IOM recommended range or secondary outcomes (Brownfoot et al 2015). Among a subset of women who provided feedback (n=586), 73% were comfortable with being weighed routinely (Brownfoot et al 2016).

A pilot study (Daley et al 2015) (n=76) combined regular weighing by midwives and advice on weight gain with self-weighing between antenatal visits. Compared to usual care, there was no clear difference in the percentage of women gaining excessive weight during pregnancy or in mean depression and anxiety scores. Feedback in a subset of participants showed support for routine weighing among participants (9/12) and midwives (7/7).

When these two trials were pooled (n=711), there was no clear difference in excessive gestational weight (RR 1.05 95% CI0.95 to 1.16) or in mean weekly weight gain (0.01 kg per week 95%CI –0.03 to 0.05). Quality of evidence was low for both outcomes. There was no indication in the two trials that either excessive gestational weight gain or mean gestational weight gain differed in women of normal weight at the beginning of pregnancy compared with women who were overweight or obese.

A third study (from Australia) found that, compared to usual care, self-weighing plus advice on weight gain reduced weight gain among women who were overweight but not among women who were normal weight or obese before pregnancy. However, the intervention did not influence excessive weight gain (n=236) (Jeffries et al 2009).

Recommendation

  • Consensus-based
  • X

At every antenatal visit, offer women the opportunity to be weighed and encourage self-monitoring of weight gain.

Approved by NHMRC in October 2017; expires October 2022 UNDER REVIEW

19.2.4 Supporting weight management

A recent meta-analysis of individual patient data from 36 RCTs, found that diet and physical activity based interventions during pregnancy reduced gestational weight gain (MD −0.70 kg; 95% CI −0.92 to −0.48 kg) and lowered the odds of caesarean section (OR 0.91; 95% CI 0.88 to 0.99) (i-WIP Collaborative Group 2017). There was no evidence that effects differ across subgroups of women (ie it is likely that women of all BMI groups could benefit from specific advice on diet and physical activity for weight management).

Recommendation

  • Consensus-based
  • XI

At every antenatal visit, discuss weight change, diet and level of physical activity with all women.

Approved by NHMRC in October 2017; expires October 2022 UNDER REVIEW

Nutrition and physical activity in pregnancy are discussed in Chapter 11.

19.2.5 Specific risk assessments required for women who are underweight or overweight/obese

A high BMI during pregnancy highlights the need to monitor fetal growth (RCOG 2014), gestational diabetes (Chu et al 2007b; Callaway et al 2006) and hypertensive disorders (Callaway et al 2006; HAPO 2008; HAPO 2010), congenital anomaly (Chu et al 2007b; Oddy et al 2009; Stothard et al 2009) and neural tube defects (Rassmussen et al 2008; Oddy et al 2009; Stothard et al 2009). Individual assessment of the risk of potential complications during the birth, including anaesthetic risk, may also be necessary for women with a BMI ≥40.

19.2.6 Other considerations

  • Potential for sub-optimal visualisation on ultrasound for women with elevated BMI (delaying the ultrasound until 20 to 22 weeks pregnancy for women with BMI ≥30 may provide better results but needs to be balanced against the possibility of a delayed diagnosis of structural anomalies) (SOGC 2010).
  • Antenatal consultation with an obstetric anaesthetist to identify any potential difficulties with venous access, regional or general anaesthesia for women with a BMI ≥40.
  • Additional support for initiating breastfeeding for women with BMIs lower or higher than the healthy range.
  • For women with a high BMI, ongoing nutritional advice following childbirth from an appropriate health professional, with a view to weight reduction and maintenance.

19.3 Practice summary: weight and BMI

When

At first antenatal visit.

Who

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

What

  • Explain the purpose of assessing weight and weight gain during pregnancy
    For women with a BMI outside the healthy range, discuss the risks associated with a woman’s weight being below or above the healthy range before, during pregnancy and in between pregnancies.
  • Engage women in discussions about weight gain
    Offer women the opportunity to be weighed and to discuss their weight gain since the last antenatal visit. Use the IOM recommendations to give women advice about the risks of inadequate or excessive weight gain, regardless of BMI. Provide advice on nutrition and lifestyle based on the Australian dietary and physical activity guidelines.
  • Take a holistic approach
    Provide women with culturally appropriate advice on the benefits of a healthy diet and regular physical activity.
  • Consider referral
    Women who are gaining weight at a rate below or above recommendations for gestational weight gain may benefit from referral for nutrition and lifestyle advice (eg from an accredited practising dietitian).

19.4 Resources

19.4.1 Health professionals

19.4.2 Women and families

References

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  • 16 Data from Victoria, Queensland, Western Australia, South Australia, Tasmania and the Australian Capital Territory.
Last updated: 
4 December 2018