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In this issue of the journal, we explore the effects of obesity on various aspects of reproductive health. Obesity is ‘the modern epidemic’. Currently, 33% of women in the UK are overweight [body mass index (BMI) >25] and 23% are obese (BMI > 30), so well over half the female population weigh more than is good for them personally or for society as a whole. The effects of obesity on health are well known to most of us. However, we are now seeing women becoming obese at a younger age, and the ramifications for reproduction, pregnancy and gynaecological disease are causes for great concern. This concern is highlighted by the fact that the Royal College of Obstetricians and Gynaecologists has recently hosted a meeting on obesity and gynaecological health and has also agreed to convene a Special Study Group to examine the causes and sequelae of obesity. The British Fertility Society has also held a recent symposium on the topic of obesity.

This issue of BJOG has contributions from many of those who have addressed the recent meetings, and also from other authors of international renown in the field. We set the scene with general reviews of the effects of obesity on the various components of female health.

Childhood obesity is already resulting in significant morbidity. Plentiful nutrition led to a secular trend of lowering the age of menarche during the 20th century. Unfortunately, excess nutrition and reduced exercise during adolescence is now increasing obesity rates and have a profound effect on the establishment of the menstrual cycle. Indeed, the origins of obesity in adulthood largely relate to childhood nutrition and lifestyle. Around the world, there are large migrant populations adapted to an intermittent food supply who do not cope easily with the ready supply of calories found in Western diets. They are at increased disease risk for many reasons, including social deprivation. The article from Austria on page 1188 (Obesity among female adolescents in Vienna, Austria—the impact of childhood weight status and ethnicity Sylvia Kirchengast and Edith Schober) emphasises these risks with respect to obesity and its long-term risks and stresses the need for educating and supporting migrant groups.

For those with polycystic ovaries, obesity worsens the expression of the polycystic ovary syndrome (PCOS) and sets the scene for prolonged reproductive disturbance. There is still debate regarding the origins of PCOS, but there is no doubt that obesity worsens the severity of symptoms and endocrine abnormalities. Included in this issue of the journal are articles that address the effects of PCOS at different ages, and the impact of obesity both during adolescence and on later reproductive health. Fat distribution is in many respects more important than total BMI, as it is increased visceral fat that correlates best with metabolic disturbance. Metformin therapy has yet to find its true place in the management of PCOS, but its potential effects on visceral fat are explored in the article by Lord et al. on page 1203 (The Central Issue? Visceral fat mass is the best marker of insulin resistance and metabolic disturbance in women with polycystic ovary syndrome).

There has been controversy and much discussion about the Rotterdam consensus criteria for defining PCOS, principally because they widened the diagnosis of this heterogeneous syndrome. In the article on page 1210 by Broekmans et al. (PCOS according to the Rotterdam consensus criteria: change in prevalence among WHO-II anovulation and association with metabolic factors), the nature of metabolic disturbance is explored in relation to the new definition and a previous National Institute of Health (USA) definition. The findings confirm an increased rate of diagnosis of PCOS, while at the same time the number with a metabolic disturbance is significantly reduced, which has implications for the overall management of these patients while at the same time illustrating the need for an individualised approach to care.

The effects of obesity on reproduction are further explored by Pasquali in his comprehensive review on page 1148 (The impact of obesity on reproduction in women with polycystic ovary syndrome) and highlighted by Farquhar (Prioritising for fertility treatments—can we justify excluding severely overweight women? The New Zealand experience) on page 1107 who presents the controversial debate about the withholding of fertility treatment to those who are severely overweight, based on the outcome both of treatment and of the subsequent pregnancy. Infertility is on the increase, and obesity has a profound effect on both natural fecundity and the likelihood of conception following treatment, thus driving increases in the demand for treatment. In obese women, fertility treatments are less successful, and higher doses of drugs are needed to stimulate ovulation. However, there are few data to indicate at what weight it might be appropriate to refuse fertility treatment. While the chance of conception may be adversely affected by being overweight, and the subsequent pregnancy will be more risky for both mother and baby, where should the cutoff be made? Such debates inform algorithms for funding and treatment.

In this special issue, we also highlight the influence of obesity on the outcome of pregnancy. Not only does obesity have numerous effects on fetal development but also consequences for intrapartum analgesia, progress of labour, and mode of delivery. There may also be longer term health consequences predicted by the development of antenatal conditions such as gestational diabetes. An overview of obesity as it affects pregnancy is provided by Krishnamoorthy et al. on page 1134 (A review of maternal obesity in pregnancy: is it time for meaningful research to inform preventive and management strategies?) and expanded in the articles by Yu (Obesity in pregnancy) and Catalano on pages 1126 (The short and long term implications of maternal obesity on the mother and her offspring). The practical considerations of managing pain relief in labour are comprehensively reviewed by Dresner et al. on page 1178 (Audit of the influence of body mass index on the performance of epidural analgesia in labour and the subsequent mode of delivery).

To supplement the articles on obesity that we have solicited, we have also collected together articles recently submitted to the journal of relevance to overweight women and their related health risks. However, whether invited or submitted spontaneously, all the articles in this issue have passed our rigorous process of peer review. By the very nature of the submission and review process, this issue of BJOG cannot (and was never intended to) provide a comprehensive review of every aspect of obesity and its effect on female health, but it does serve to emphasise the importance of the topic and highlight a number of areas for further debate and discussion.