International prevalence

International studies show a prevalence of maternal obesity ranging from 1.8% to 25.3% across countries (using the WHO definition of obesity of BMI of at least 30kg/m2) [1]. However, there are difficulties when trying to compare international rates using data from published studies due to differences in BMI categories used to define obesity, differences in study time periods and a preponderance of hospital-based as opposed to population-based studies. Also, most publications are based on studies conducted in the United States and Australia and are thus not broadly internationally representative[1].

UK prevalence

National statistics for the prevalence of maternal obesity are not collected routinely in the UK.

The UK-wide Obstetric Surveillance System (UKOSS) carried out a surveillance study of extreme obesity during pregnancy between March 2007 and August 2008. The study identified that nearly one in every thousand women delivering in the UK has a body mass index (BMI) of at least 50kg/m2 or weighs more than 140kg [2].

The Centre for Maternal and Child Enquiries (CMACE), formerly known as the Confidential Enquiry into Maternal and Child Health (CEMACH), has conducted a national audit of obesity during pregnancy. Between 1 March and 30 April 2009, all maternity units in the UK, Channel Islands and Isle of Man prospectively identified and notified CEMACH of women delivering from 24 weeks’ gestation who met the following criteria:

• BMI of at least 35 kg/m2 at any time during pregnancy
• BMI not known, but weight at least 100kg at any time during pregnancy
• BMI or weight not known, but judged by health professionals to be in one of the above categories.

The results of the audit are due to be published in 2010/11. Prevalence figures will be presented separately for women with a BMI of at least 35kg/m2 at booking and those with a BMI of at least 35kg/m2 at any point during pregnancy.

Trends in UK prevalence

Trend data from the Health Survey for England (HSE) for the period 1993 to 2008 (Figure 1) show an increase in the prevalence of obesity (BMI at least 30 kg/m2) from 14.4% in 1997 to 20.2% in 2008 amongst women of childbearing age (16 to 44 years)[3]. Within this age group, the prevalence of obesity also increases with age; in 2008, obesity prevalence was 14% in those aged 16-24 and 25.0% in those aged 35-44.

Figure 1: Prevalence of obesity (with 95% confidence intervals) in females aged 16-44 years during the period 1997-2008

Source: Health Survey for England

Three studies conducted in the UK and published within the last five years have shown an increase in the prevalence of obesity amongst pregnant women presenting to hospital for booking. In these studies, the prevalence of obesity increased as follows:–

Demography and health inequalities

The demographic characteristics of women who are obese during pregnancy are similar to those of obese women in the general UK population, with a similar socioeconomic gradient. Women who are obese are significantly more likely to be older in pregnancy and to have a higher parity, and are more than twice as likely to live in areas of high deprivation, compared with women who are not obese [5]. The UKOSS study for extreme obesity similarly found that pregnant women with a BMI of at least 50kg/m2 were likely to be older than average, white, multiparous, and from routine and manual social groups [2].