Teenagers who are either underweight or obese are likely to have fewer children in adulthood, a study has found.

It's 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.

For the new study, researchers used data on nearly 1,300 Finnish men and women who were part of a larger study that has tracked their health since 1980. All were between the ages of 3 and 18 at the study's outset, and had their body mass index (BMI) measured in adolescence.

The researchers report the findings in the medical journal Epidemiology. Overall, adults who had been underweight as teenagers had 10 per cent to 16 per cent fewer children, compared with those who had a normal BMI in adolescence. Men and women who'd been obese as teens had 32 per cent to 38 per cent fewer children.

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