Introduction

The effect of obesity in pregnancy is wide ranging with potentially serious impacts on both the mother and the child. This chapter summarizes the current state of knowledge of the relationship between obesity and adverse maternal outcome, including effects on the health of the pregnant mother as well as complications at the time of delivery. We examine how obesity influences the risk of maternal death, gestational diabetes, hypertensive diseases of pregnancy, and infection. In regard to delivery we summarize evidence for effects on induction of labor, cesarean section (CS), postpartum hemorrhage, and other serious complications. This chapter draws together a considerable body of evidence to show that risks of adverse obstetric outcome may be doubled or even trebled in an obese woman compared to a woman with a healthy body mass index (BMI). The final section of the chapter looks at the impact of maternal obesity at a population, rather than an individual, level. We highlight the proportion of adverse obstetric outcome that is explained by obesity in the population and estimate that approximately one fifth of obstetric morbidity in developed countries could be avoided if maternal obesity was eliminated.

What is obesity in pregnancy?

Measurement and definition of obesity

Obesity is defined as “an accumulation of excess body fat to such an extent that may impair health” [1]. Total body fat can be measured by direct methods such as dual energy X-ray absorptiometry (DEXA) and magnetic resonance imaging (MRI) [2]. Both are expensive, cumbersome, and impractical to perform in most circumstances. Moreover, DEXA has the added radiation risk [3]. Hence obesity is usually measured using indirect methods such as BMI, based on anthropometry. Body mass index is an expression of body weight-for-height using the formula weight (in kilograms) divided by height (in meters) squared (kg/m2). Obesity in adults is defined by the World Health Organization (WHO), the US Institute of Medicine (IOM), and the UK National Institute for Health and Clinical Excellence (NICE) as BMI ≥30kg/m2. Table 1.1 of Chapter 1 describes the different classifications of obesity in common usage.