Super obesity

[Completed study: see Sullivan et al: Maternal super-obesity and perinatal outcomes in Australia: a national population-based cohort study]


The prevalence of obesity has risen dramatically in Australia and has increasingly become an important public health concern. A reported 35% of Australian women between the ages of 25-35, (equating to 57,983 of women who gave 6) are either overweight or obese 1. Obesity in pregnancy is an independent risk factor for obstetric complications. The most consistently described maternal complications during pregnancy and delivery of obese women include: hypertensive disorders of pregnancy, (for example Fredrick et al found that every unit increase in pre-pregnancy BMI resulted in an 8% increase in pre-eclampsia 35); gestational diabetes, (with 70% of obese women with gestational diabetes are reported to develop type 2 diabetes within 15 years of delivery compared with 30% of lean women 4) caesarean section rates and longer maternal stays are also reported to increase significantly 2. What’s more, maternal obesity can have practical implications for maternity service providers; examples include appropriately designed moving and handling apparatus, hospital beds, operating tables, lifting patients and anaesthetic procedures and outcomes. We do not know the prevalence of morbid obesity with a BMI >50 in the Australian obstetric population, nor do we have a clear understanding of the risk factors, treatment and outcomes associated with a BMI >50 on the pregnant women and the infant.

Research questions

  1. What is the prevalence of morbid obesity (BMI>50) during pregnancy in Australia and New Zealand?
  2. What additional management and difficulties arise in the care of pregnant women who are morbidly obese in Australia?
  3. What are the outcomes for both the mother and the infant of women who are morbidly obese during pregnancy in Australia?


Prospective prevalence study using monthly negative surveillance system of all birthing services in Australia and New Zealand (>50 births) – AMOSS. Nominated clinicians and midwives within each maternity unit were e-mailed a simple tick-box to indicate whether a case occurred or whether there is ‘nothing to report’. If a case arose, the reporting clinician (Australia only) completed a case form using the secure web-based data system. Only non-identifiable data were collected.

Study period

January 2010 - December 2010 (surveillance ANZ); January-October 2010 (observational study Australia)

Case definition

Body mass index (BMI) is an index of weight-for-height that is commonly used to classify overweight and obesity in adult populations. The World Health Organization (WHO) defines "overweight" as a BMI equal to or more than 25, and "obesity" as a BMI equal to or more than 30. This study will concentrate on women who have a calculated BMI >50 at any point during pregnancy; or a weight over 140Kg at any point during pregnancy; and/or estimated to be in either of the above categories but weight exceeds the capacity of hospital scales

Sample size

Surveillance was conducted for twelve months, and the observational study in Australia ran for ten months Jan-Oct 2010. The estimated sample size for this period was between 260-400 cases. This is based upon the incidence found in the UK of 9.1/10,000 women giving birth, who had a BMI >50 (95% CI 8.1-10.1 per 10,000). However, a higher number of cases was anticipated due to the increasing prevalence of severe obesity in Australia; a study conducted at a major Queensland centre found 3 in 1,000 pregnant women had a BMI >50 2; three times higher than that found in the UK. 



Professor Elizabeth Sullivan, Principal Investigator AMOSS, Professor of Public Health, Assistant Deputy Vice Chancellor, University of Technology Sydney and Conjoint Professor, UNSW Medicine, New South Wales

Professor David Ellwood, CI AMOSS, Director of Maternal-Fetal Medicine, Professor of Obstetrics & Gynaecology, Deputy Head of School Research, Gold Coast Health District, Queensland and Deputy Dean, College of Medicine, Australian National University, Australian Capital Territory

Professor Michael Peek, CI AMOSS, Associate Dean, Department of Obstetrics and Gynaecology, Medical School, College of Medicine, Biology and Environment, The Australian National University and Centenary Hospital for Women and Children, Australian Capital Territory

Professor Caroline Homer, CI AMOSS, Professor of Midwifery, Associate Dean: International and Development, Associate Head, WHO Collaborating Centre for Nursing, Midwifery and Health Development, Faculty of Health, University of Technology Sydney, New South Wales

Professor Lisa Jackson Pulver, CI AMOSS, ProVice Chancellor Engagement and Aboriginal and Torres Strait Islander Leadership, Western Sydney University, New South Wales 

Professor Marian Knight, NIHR Professor of Maternal and Child Population Health, National Perinatal Epidemiology Unit (NPEU), University of Oxford, UK.

Dr. Claire McLintock, CI AMOSS, Obstetric physician and haematologist, National Women's Health, Auckland City Hospital, New Zealand


Professor Elizabeth Elliott, Professor of Paediatrics & Child Health, The University of Sydney and Australian Perinatal Statistics Unit, The Children's Hospital Westmead New South Wales

Clinical Assoc Professor Nolan McDonnell, Staff Specialist Department of Anaesthesia and Pain Medicine, King Edward Memorial Hospital for Women, Perth Western Australia

Dr Tessa Ho, Trustee, Mary Aitkenhead Ministries

Dr Wendy Pollock, Honorary Research Fellow, La Trobe University/Mercy Hospital for Women, Melbourne Victoria

Associate Professor Yvonne Zurynski, The University of Sydney and Australian Perinatal Statistics Unit, The Children's Hospital Westmead New South Wales

Funding and other support

We gratefully acknowledge our funding partners and participating AMOSS sites.

Initial funding was through a five year project grant (Australian National Health and Medical Research Council NHMRC #510298 2008-2012), which provided support for the first set of AMOSS studies as well as the AMOSS infrastructure.

The University of New South Wales provided support to redevelop the AMOSS site with a Major Research Equipment and Infrastructure Initiative (MREII) Grant in 2013.

Other funding support has been provided through the University of Technology Sydney.

Data collection is supported by AMOSS participating sites.


  1. Cameron, AJ, Welborn TA Zimmer PZ et al Overweight and obesity in Australia the 1999-2000 Australian diabetes Obesity and Lifestyle study (Ausdiab) Med J Aust 2003 178: 427-432
  2. Callaway LK, Johannes BP Chang AM & McIntyre D, The prevalence and impact of overweight and obesity in an Australian obstetric population. MJA Vol 184 No 2. 2006
  3. Frederick IO, Rudra CB, Miller RS, Foster JC, Williams MA, Adult weight change, weight cycling and prepregnancy obesity in relation to risk of pre-eclampsia. Epidemiology 2006 17:428-434
  4. O’Sullivan JB, Subsequent morbidity among gestational diabetic women. In Sutherland HW Stowers JM (eds) Carbohydrate Metabolism in Pregnancy and the Newborn. Churchill livingstone Edinburgh 1984 pp174-180
  5. O'Brien TE, Ray JG, Chan WS. Maternal body mass index and the risk of preeclampsia: a systematic overview. Epidemiology. 2003 May;14(3):368-74.
  6. Laws PJ & Hilder, L 2008 Australian Mothers and Babies 2006 Perinatal statistics series no 22 PER 46 Sydney AIHW National Perinatal Statistics Unit