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Extreme Obesity

Research questions

  • What is the current prevalence of extreme obesity in pregnancy in the UK?
  • What is the prevalence among different racial and socioeconomic groups?
  • What are the risks of adverse outcomes for mother and infant associated with extreme obesity in pregnancy?
  • Are any adverse outcomes associated with inadequate weight capacity equipment?


Obesity is now recognised to be an important public health problem throughout the developed world[1]. The prevalence of obesity is rising rapidly in the UK in all age groups, including women of reproductive age[2]. Recent reports of the UK Confidential Enquiry into Maternal Deaths[3]have highlighted obesity as a factor in increasing numbers of maternal deaths in the UK. Retrospective database analyses in Canada, Australia and the UK have identified particular disease risks associated with pregnancy among obese women[4][5][6] , including pre-eclampsia, venous thromboembolism and gestational diabetes, and higher rates of labour induction, delivery by caesarean section, general anaesthesia and anaesthetic complications[7]. Obese women are also at increased risk of poor perinatal outcomes, including stillbirth and neonatal death[8]. The majority of these studies focus on women with moderate obesity (BMI greater than 30). The studies therefore include only a very few women who are extremely obese and have not specifically addressed the risks in the extremely obese group. The risk of pregnancy complications in extremely obese women is potentially even higher than among moderately obese women. However, because of the relatively small numbers of women with this degree of obesity, a national study is needed to investigate this further.

Case definition

All women in the UK identified as having:

EITHER any woman weighing over 140Kg at any point during pregnancy

OR any woman with a Body Mass Index (BMI) greater than 50 at any point during pregnancy

OR any woman estimated to be in either of the previous categories but whose weight exceeds the capacity of hospital scales.

Study size

Extrapolation from the Millennium Cohort Study suggests the study may expect to identify 200-300 cases over a one year period (based on 9 extremely obese women in a cohort of 18,000).

Download the Data Collection Form (DCF)

UKOSS Extreme Obesity Phase B Form


  1. ^ World Health Organisation. Controlling the global obesity epidemic,2006. Available at (Accessed July 2006).
  2. ^ Department of Health. Health Survey for England 2004.
  3. ^ Confidential Enquiry into Maternal and Child Health. Why women die 2000-2002. London: Royal College of Obstetricians and Gynaecologists, 2004.
  4. ^ Robinson HE, O'Connell CM, Joseph KS, McLeod NL. Maternal outcomes in pregnancies complicated by obesity. Obstet Gynecol 2005; 106(6):1357-64.
  5. ^ Usha Kiran TS, Hemmadi S, Bethel J, Evans J. Outcome of pregnancy in a woman with an increased body mass index. BJOG 2005; 112(6):768-72.
  6. ^ Callaway LK, Prins JB, Chang AM, McIntyre HD. The prevalence and impact of overweight and obesity in an Australian obstetric population. Med J Aust 2006; 184(2):56-9.
  7. ^ Catalano P, Ehrenberg H. The short- and long-term implications of maternal obesity on the mother and her offspring. BJOG 2006.
  8. ^ Kristensen J, Vestergaard M, Wisborg K, Kesmodel U, Secher NJ. Pre-pregnancy weight and the risk of stillbirth and neonatal death. BJOG 2005; 112(4):403-8.

Updated: Tuesday, 17 January 2023 17:56 (v8)