Obesity is an increasingly prevalent health burden upon modern society. Most obese women are not infertile; however, obesity and its negative impact upon fecundity and fertility are well documented. Obese women are three times more likely to suffer infertility than women with a normal body mass index (BMI; Rich-Edwards et al. 1994). Obese women experience impaired fecundity both in natural and assisted conception cycles (Zaadstra et al. 1993, Crosignani et al. 1994). The mechanism through which its effect is exerted is more controversial. Obesity is characterised by excess lipid storage. Definitions of obesity can vary, but the most widely accepted definition adheres to the WHO BMI (kg/m2) criteria. A person is obese if his/her BMI is ≥30 kg/m2. There are degrees of obesity: class 1 (30.0–34.9 kg/m2), class 2 (35.0–39.9 kg/m2) and class 3 (≥40 kg/m2). Alternative, although less commonly used, parameters for the assessment of obesity include waist circumference and waist:hip ratio (WHR). A waist circumference >80 cm in women is an accepted indicator of visceral fat accumulation (Tamer Erel & Senturk 2009).
Population studies demonstrate an increasing prevalence of overweight and obese women, and maternal obesity detected an increase from 9.9 to 16.0% from 1990 to 2005 (Lewis 2007, Heslehurst et al. 2007). It has been proposed that obesity impairs fertility through an effect upon the control of ovulation, oocyte development, embryo development, endometrial development, implantation and pregnancy loss. This article aims to summarise the current understanding of how obesity might affect these processes in the context of natural and assisted conception