Clinical Practice Guidelines Antenatal care - Module I

7.2 Weight and body mass index

Page last updated: 02 April 2013

Pre-pregnancy weight and weight gain during pregnancy are important determinants of the health of both mother and baby.

7.2.1 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.

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 World Health Organization classification of body mass index classification is given in Table 7.2.

Table 7.2: International classification of adult underweight, overweight and obesity according to BMI

Underweight–healthy range
BMI
Overweight–obese
BMI
Underweight <18.50 Overweight ≥25.00
Severe thinness <16.00 Pre-obese 25.00–29.99
Moderate thinness 16.00–16.99 Obese ≥30.00
Mild thinness 17.00–18.49 Class I 30.00–34.99
Class II 35.00–39.99
Healthy weight 18.50–24.99 Class III
≥40.00
Source: Adapted from WHO 1995, WHO 2000 and WHO Expert Consultation 2004.

Prevalence of low or high BMI in Australia

  • In 2007–08, 3.4% of women aged 25–34 were underweight, 26.5% were overweight and 18% were obese (AIHW 2010). In the 35–44 age group, 1.9% of women were underweight, 32.5% overweight and 22.8% obese (AIHW 2010). In 2004–05 age-adjusted rates of overweight/obesity were similar for Aboriginal and Torres Strait Islander and non-Indigenous adults. However, Aboriginal and Torres Strait Islander women were around one-and-a-half times more likely than non-Indigenous women to be overweight/obese (AIHW 2008).
  • A retrospective analysis of women receiving antenatal care at an urban tertiary maternity hospital in Brisbane (n=14,230) found that 21% had a BMI ≤20, 45% had a BMI within the healthy range and 34% were classified as overweight, obese or morbidly obese (Callaway et al 2006).
  • An RCT involving women attending antenatal care in Melbourne (n=236) found that 4.2% of women were underweight, 60.2% had a BMI within the healthy range, 16.1% were classified as overweight and 19.5% were classified as obese (Jeffries et al 2009).
  • A Swedish cohort study (n=298,648)(Cedergren 2007) found a total prevalence of overweight and obesity of 33% among pregnant women, with 10.7% of women classified as obese.

Risks associated with a low or high pre-pregnancy BMI

  • Underweight — a low pre-pregnancy BMI is associated with increased risk of preterm birth (Siega-Riz et al 1996; Panaretto et al 2006; Khashan & Kenny 2009) and small-for-gestational-age babies (Dawes & Grudzinskas 1991; Panaretto et al 2006). 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 — pre-pregnancy BMI >25 has been linked with stillbirth (Chu et al 2007a), congenital abnormalities (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) and major depressive disorders (Bodnar et al 2009).
  • Obesity — pre-pregnancy BMI ≥30 is also linked to an inability to initiate breastfeeding (Viswanathan et al 2008), postpartum weight retention (Thornton et al 2009) and increased rate of caesarean birth (Callaway et al 2006; Chu et al 2007c; HAPO 2010).
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Risks associated with low or high weight gain during pregnancy

  • Low weight gain in pregnancy is associated with small-for gestational-age babies (DeVader et al 2007; Nohr et al 2008; Viswanathan et al 2008), preterm birth (Siega-Riz et al 1996; Viswanathan et al 2008), low birth weight and an inability to initiate breastfeeding (Viswanathan et al 2008).
  • High weight gain in pregnancy increases the risk of large-for-gestational-age babies (DeVader et al 2007; Nohr et al 2008; Viswanathan et al 2008; Siega-Riz et al 2009), high birth weight (Viswanathan et al 2008; Crane et al 2009) and caesarean birth (DeVader et al 2007; Nohr et al 2008; Viswanathan et al 2008; Bodnar et al 2010). It is also associated with hypertension (Crane et al 2009) and pre-eclampsia (DeVader et al 2007). High weight gain in women who are obese has been associated with neonatal metabolic abnormalities (Crane et al 2009). Weight gain before, during, and after 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).

7.2.2 Assessing BMI and weight gain

Summary of the evidence

Routinely measuring women’s height and weight and calculating BMI at an early antenatal contact is recommended in the United Kingdom (NICE 2008; CMACE & RCOG 2010), the United States (IOM 2009) and Canada (AOM 2010; SOGC 2010). Additional advice on monitoring weight gain during pregnancy, in particular for women who are underweight, overweight or obese is also given in the Canadian and United States guidelines.

Measuring height and weight and calculating BMI

Recent evidence on the risks associated with low or high BMI during pregnancy (see Section 7.2.1) continues to support the routine measurement of women’s weight and height and calculation of BMI at the first antenatal contact. This allows identification of women who require additional care during pregnancy. 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 Asian women and higher cut-offs are recommended for women from Pacific Islands)(Duerenberg et al 2002; James et al 2004; Depres & Tchernof 2007).

Recommendation - Grade B

3. Measure women’s weight and height at the first antenatal visit and calculate their BMI.

Monitoring weight gain during pregnancy

While pre-pregnancy BMI 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 amount of weight gained during pregnancy varies and studies suggest that many women do not gain the amount recommended. In a large cohort study in the United States (n=94,696)(DeVader et al 2007), 17.8% of women gained less than the recommended amount of weight over the pregnancy, 39.4% gained the recommended amount and 42.8% gained more than recommended. Similar results were found in a Canadian cohort study (5,377)(Crane et al 2009), with 17.1% of women gaining less than the recommended amount, 30.6% gaining the recommended amount and 52.3% gaining more weight than recommended.

The US Institute of Medicine (IOM) provides guidance on weight gain in pregnancy based on prepregnancy BMI (see Table 7.3). When pre-pregnancy BMI is not known, a weight gain of 0.5–2 kg weight gain in the first trimester may be assumed (IOM 2009).

Table 7.3: IOM recommendations for weight gain in pregnancy by pre-pregnancy BMI

<18.5 18.5 to 24.9 25.0 to 29.9 30.0 to 34.9 35.0 to 39.9 ≥40
12.7–18.1kg 11.3–15.9kg 6.8–11.3kg 5–9kg 5–9kg 5–9kg
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Practice point

f. Repeated weighing during pregnancy should be confined to circumstances that are likely to influence clinical management.

Supporting weight management

Two systematic reviews examining interventions to reduce excessive weight gain in pregnancy (Dodd et al 2010; Ronnberg et al 2010) found insufficient evidence to support specific interventions. A meta-analysis (Streuling et al 2010) however, found that interventions based on physical activity and dietary counselling, usually combined with supplementary weight monitoring, appear to be successful in reducing inappropriate weight gain in pregnancy. An RCT (n=236)(Jeffries et al 2009) found that self-monitoring of weight gain was effective in reducing excessive weight gain among women who were overweight but not obese before pregnancy, but not effective for women who were normal weight or obese before pregnancy. Protein/energy restriction of pregnant women who are overweight, or exhibit high weight gain, is unlikely to be beneficial and may harm the baby (Kramer & Kakuma 2003).

Dietary advice has been shown to be effective in increasing energy and protein intake in pregnant women but has no major benefit on the health of the mother or baby (Kramer & Kakuma 2003). Balanced energy/ protein supplementation improves fetal growth and may reduce the risk of fetal and neonatal death, however high-protein or balanced-protein supplementation alone is not beneficial and may be harmful to the fetus (Kramer & Kakuma 2003).

Recommendation - Grade B

4. Give women advice about appropriate weight gain during pregnancy in relation to their BMI.

Specific risk assessments required for pregnant women above and below their most healthy weight.

There is strong evidence to support assessment of risks associated with a high pre-pregnancy BMI, including monitoring fetal growth (HAPO 2010) and checking for gestational diabetes (Chu et al 2007b; Callaway et al 2006) and hypertensive disorders (Callaway et al 2006; HAPO 2008; 2010), congenital abnormality (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 BMI ≥40. There is also evidence to support monitoring for small-for-gestational-age babies for women with a pre-pregnancy BMI in the underweight category (Dawes & Grudzinskas 1991; Panaretto et al 2006).

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 a high 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 prepregnancy BMI should be explained and the woman given the opportunity to discuss these and how they might be minimised.

Practice point

g. Taking 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.

Considerations beyond the first trimester

  • Potential for sub-optimal visualisation on second trimester ultrasounds (20 to 22 weeks pregnancy may provide better results in women with BMI ≥30).
  • 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.

7.2.3 Practice summary — measuring weight and BMI

When — At first antenatal visit
Who — Midwife; GP; obstetrician; Aboriginal and Torres Strait Islander health worker; multicultural health worker
  • 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.
  • Estimate pre-pregnancy weight — As many women do not attend for antenatal care until later in the first trimester, estimation of pre-pregnancy BMI may be based on self-reported weight.
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7.2.4 Resources

Health professionals
AOM (2010) Ontario Midwives Clinical Practice Guideline No. 12 The Management of Women with a High or Low Body Mass Index. Toronto: Association of Ontario Midwives.
CMACE & RCOG (2010) CMACE & RCOG Joint Guideline. Management of Women with Obesity in Pregnancy. London: Centre for Maternal and Child Enquiries & Royal College of Obstetricians and Gynaecologists.

IOM (2009) Nutrition During Pregnancy. National Academy of Sciences, Institute of Medicine, Food and Nutrition Board, Committee on Nutritional Status During Pregnancy and Lactation, Subcommittee on Dietary Intake and Nutrient Supplements During Pregnancy, Subcommittee on Nutritional Status and Weight Gain During Pregnancy. Washington DC: National Academy Press; 1990.

National Health and Medical Research Council (2013) Australian Dietry Guidelines. Canberra: National Health and Medical Research Council.

NHMRC (2005) Nutrient Reference Values for Australia and New Zealand. Canberra: National Health and Medical Research Council. For more information please visit National Health and Medical Research Council website.

NICE (2010) Dietary Interventions and Physical Activity Interventions for Weight Management Before, During and After Pregnancy. NICE public health guidance 27. London: National Institute for Health and Clinical Excellence.

SOGC (2010) Obesity in pregnancy. J Obstet Gynaecol Can 32(2): 165–73.

DoHA lifescripts:

Nutrition and weight gain. For more information please visit Department of Health and Ageing website.

Physical activity. For more information please visit Department of Health and Ageing website.

Women and families
DoHA (1998) An Active Way to Better Health. National Physical Activity Guidelines for Adults. Canberra: Australian Government Department of Health and Ageing. For more information please visit Department of Health and Ageing website.

DoHA (1998) Australian Guide to Healthy Eating. Background Information for Consumers. Canberra: Australian Government Department of Health and Ageing. [under review]. For more information please visit Department of Health and Ageing website.

7.2.5 References

AIHW (2008) The Health and Welfare of Australia’s Aboriginal and Torres Strait Islander Peoples. ABS Cat No 4704.0, AIHW Cat No IHW 21. Commonwealth of Australia.

AIHW (2010) Australia’s Health 2010. Australia’s health series no. 12. Cat. no. AUS 122. Canberra: Australian Institute of Health and Welfare.

AOM (2010) Ontario Midwives Clinical Practice Guideline No. 12 The Management of Women with a High or Low Body Mass Index. Toronto: Association of Ontario Midwives.

Bodnar LM, Siega-Riz AM, Simhan HN et al (2010) Severe obesity, gestational weight gain, and adverse birth outcomes. Am J Clin Nutr 91(6): 1642–48.

Bodnar LM, Wisner KL, Bodnar LM et al (2009) Prepregnancy body mass index, gestational weight gain, and the likelihood of major depressive disorder during pregnancy. J Clin Psychiatry 70(9): 1290–96.

Callaway LK, Prins JB, Chang AM et al (2006) The prevalence and impact of overweight and obesity in an Australian obstetric population. Med J Aust 184(2): 56–59.

Cedergren MI (2007) Optimal gestational weight gain for body mass index categories. Obstet Gynecol 110(4): 759–64.

Chu SY, Kim SY, Lau J et al (2007a) Maternal obesity and risk of stillbirth: a metaanalysis. Am J Obstet Gynecol 197(3) 223–38.

Chu SY, Callaghan WM, Kim SY et al (2007b) Maternal obesity and risk of gestational diabetes mellitus. Diabetes Care 30(8): 2070–76.
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Chu SY, Kim SY, Schmid CH et al (2007c) Maternal obesity and risk of cesarean delivery: a meta-analysis. Obes Rev 8(5): 385–94.

CMACE & RCOG (2010) CMACE & RCOG Joint Guideline. Management of Women with Obesity in Pregnancy. London: Centre for Maternal and Child Enquiries & Royal College of Obstetricians and Gynaecologists.

Crane JM, White J, Murphy P et al (2009) The effect of gestational weight gain by body mass index on maternal and neonatal outcomes. J Obstet Gynaecol Can 31(1): 28–35.

Dawes MG & Grudzinskas JG (1991) Repeated measurement of maternal weight during pregnancy. Is this a useful practice? Brit J Obstet Gynaecol 98: 189–94.

Depres JP & Tchernof A (2007) Classification of overweight and obesity in adults. In: Lau DCW, Douketis JD, Morrison KM, et al (eds) 2006 Canadian Clinical Practice Guidelines on the Management and Prevention of Obesity in Adults and Children. Can Med Assoc J 176: 21–26.

Deurenberg P, Deurenberg-Yap M, Guricci S (2002) Asians are different from Caucasians and from each other in their body mass index/body fat per cent relationship. Obesity Rev 3(3): 141–46.

DeVader SR, Neeley HL, Myles TD et al (2007) Evaluation of gestational weight gain guidelines for women with normal prepregnancy body mass index. Obstet Gynecol 110(4): 745–51.

Dodd JM, Grivell RM, Crowther CA et al (2010) Antenatal interventions for overweight or obese pregnant women: a systematic review of randomised trials. Brit J Obstet Gynaecol 117(11): 1316–26.

HAPO Study Cooperative Research Group (2008) Hyperglycemia and adverse pregnancy outcomes. N Engl J Med 358 : 1991–2002.

HAPO Study Cooperative Research Group (2010) Hyperglycaemia and Adverse Pregnancy Outcome (HAPO) Study: associations with maternal body mass index. Brit J Obstet Gynaecol 117(5): 575–84.

IOM (2009) Nutrition During Pregnancy. National Academy of Sciences, Institute of Medicine. Washington DC: National Academy Press.

James WPT, Jackson-Leach R, NiMhurchu C et al (2004) Overweight and obesity (high body mass index). In: Ezzati M, Lopez A, Rodgers A, et al (eds) Comparative Quantification of Health Risks: Global and Regional Burden of Disease Attributable to Selected Major Risk Factors. Geneva: World Health Organization, pp. 497–596.

Jeffries K, Shub A, Walker SP et al (2009) Reducing excessive weight gain in pregnancy: A randomised controlled trial.Med J Aust 191(8): 429–33.

Khashan AS & Kenny LC (2009) The effects of maternal body mass index on pregnancy outcome. Eur J Epidemiol 24(11): 697–705.

Kramer MS & Kakuma R (2003) Energy and protein intake in pregnancy. Cochrane Database Syst Rev(4): CD000032.

McDonald SD, Han Z, Mulla S et al (2010) Knowledge Synthesis Group Overweight and obesity in mothers and risk of preterm birth and low birth weight infants: systematic review and meta-analyses. Brit Med J 341: c3428.

NICE (2008) Antenatal Care. Routine Care for the Healthy Pregnant Woman. National Collaborating Centre for Women’s and Children’s Health. Commissioned by the National Institute for Health and Clinical Excellence. London: RCOG Press.

Nohr EA, Vaeth M, Baker JL et al (2008) Combined associations of prepregnancy body mass index and gestational weight gain with the outcome of pregnancy. Am J Clin Nutr 87(6): 1750–59.

Oddy WH, De Klerk NH, Miller M et al (2009) Association of maternal pre-pregnancy weight with birth defects: evidence from a case-control study in Western Australia. Aust N Z J Obstet Gynaecol 49(1): 11–15.

Panaretto K, Lee H, Mitchell M et al (2006) Risk factors for preterm, low birth weight and small for gestational age birth in urban Aboriginal and Torres Strait Islander women in Townsville. Aust NZ J Public Health 30: 163–70.

Rasmussen SA, Chu SY, Kim SY et al (2008) Maternal obesity and risk of neural tube defects: a metaanalysis. Am J Obstet Gynecol 198(6): 611–19.

Ronnberg AK, Nilsson K, Ronnberg AK et al (2010) Interventions during pregnancy to reduce excessive gestational weight gain: A systematic review assessing current clinical evidence using the Grading of Recommendations, Assessment, Development and Evaluation (GRADE)
system. Brit J Obstet Gynaecol 117 (11): 1327–34.

Siega-Riz AM, Adair LS, Hobel CJ (1996) Maternal underweight status and inadequate rate of weight gain during the third trimester of pregnancy increases the risk of preterm delivery. J Nutrition 126: 146–53.

Siega-Riz AM, Viswanathan M, Moos MK et al (2009) A systematic review of outcomes of maternal weight gain according to the Institute of Medicine recommendations: birthweight, fetal growth, and postpartum weight retention. Am J Obstet Gynecol 201(4): 339.e1–14.
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SOGC (2010) Obesity in pregnancy. J Obstet Gynaecol Can 32(2): 165–73.

Stothard KJ, Tennant PW, Bell R et al (2009) Maternal overweight and obesity and the risk of congenital anomalies: a systematic review and meta-analysis. J Acad Med Assoc 301(6): 636–50.

Streuling I, Beyerlein A, von Kries R (2010) Can gestational weight gain be modified by increasing physical activity and diet counseling? A meta-analysis of interventional trials. Am J Clin Nutr 92(4): 678–87.

Thornton YS, Smarkola C, Kopacz SM et al (2009) Perinatal outcomes in nutritionally monitored obese pregnant women: a randomized clinical trial. J Natl Med Assoc 101(6): 569–77.

Viswanathan M, Siega-Riz AM, Moos M-K et al (2008) Outcomes of maternal weight gain. Evid Rep Technol Assess 168: 1–223.

WHO (1995) Physical Status: the Use and Interpretation of Anthropometry. Report of a WHO Expert Committee. WHO Technical Report Series 854. Geneva: World Health Organization.

WHO (2000) Obesity: Preventing and Managing the Global Epidemic. Report of a WHO Consultation. WHO Technical Report Series 894. Geneva: World Health Organization.

WHO expert consultation (2004) Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet 363(9403): 157–63.