Appropriate maternal weight gain in singleton and twin pregnancies: what is the evidence?

A review of the literature on maternal weight gain in singleton and twin pregnancies has identified two main messages. Firstly, that very little robust evidence exists on which to base advice to mothers about appropriate weight gain during pregnancy. Evidence that does exist has tended to be observational, or based on under-powered intervention studies and unable to provide definitive conclusions. Secondly, that although specific weight gains have been recommended by the Institute of Medicine (IOM) in the USA, many women who gained weight outside these guidelines apparently achieved successful pregnancy and birth outcomes, in part because there were numerous confounders. Crucially, resources need to be invested to address women's concerns with weight management, so that they can receive the necessary support from midwives and others who care for them. Currently, women tend to receive little advice on weight gain during pregnancy and it is vital they are made aware of the increased risks associated with obesity, and that opportunities to minimise complications should be available and accessible prior to and during pregnancy. Overall, these findings highlight the difficulties in drawing up recommendations for individual women, whether expecting twins or singletons, on the basis of studies of populations, and the distinct difference between the USA and the UK in how weight gain is rationalised.