BMI in adolescence affects fertility

People who are either underweight or obese as teenagers are likely to have fewer children when they grow up. Body weight affects reproduction and related behaviours. It is known that both obesity and abnormally low body weight are related to reproductive difficulties, and that obesity raises the risk of a number of pregnancy complications. However, it has not been clear whether underweight and obese teenagers go on to have fewer children than their normal-weight peers. To understand the relation of body weight in adolescence with fertility in adulthood, researchers from the University of Helsinki used data on nearly 1,300 Finnish men and women who were part of a larger study (the Cardiovascular Risk in Young Finns) that has tracked their health since 1980. All participants were 12, 15 or 18 years at the start of the study, and had their body mass index (BMI) measured in adolescence. Overall, adults who had been underweight as teenagers had 10–16 per cent fewer children, compared with those who had a normal BMI in adolescence. Men and women who had been obese in their teens had 32–38 per cent fewer children. The findings showed that there was a long-term effect of teenage BMI, independent of adulthood BMI. This was particularly true of women. For women, an abnormally low BMI can disrupt the menstrual cycle, while obesity can lead to fertility problems. Obesity can, for example, contribute to polycystic ovary syndrome, a cause of infertility. Age at which menstruation began (menarche) also predicted the number of children, such that women with early or late menarche had more children than those with average age at menarche. For men, both obesity and a low BMI have been linked to poor sperm quality, and obesity can raise the risk of erectile dysfunction. Another outcome of the study was that underweight and obese teenagers were also less likely than their normal-weight peers to live with a partner in adulthood — a difference that partially explained why they had fewer children. The above-mentioned potential long-term effects of obesity at a young age show that there is more than one reason why one should prevent excessive weight gain in childhood.