How much you weigh during adolescence could determine your fertility later in life.

A Finnish study found that both underweight and obese adults had fewer children — 10 to 16 per cent and 32 to 38 per cent fewer, respectively.

One reason is that women with low BMI (Body Mass Index) have irregular menstrual cycles. Low BMI in men is linked to poor sperm quality. At the other extreme, obesity raises the risk of erectile dysfunction and is a factor in polycystic ovary syndrome, which can cause infertility.

A total of 1,300 adults participated in the study. Researchers monitored their health for 27 years, starting from their adolescence.

The study concluded that a person's BMI in the formative years had long-lasting effects that carried over into adulthood. KK Women's and Children's Hospital's Dr Ben Choey, registrar at the Department of General Obstetrics and Gynaecology, agreed with the findings.

He said: "Epidemiological studies have shown that obesity and low body weight each accounts for 6 per cent of infertility cases. Thus 12 per cent of primary infertility results from deviations in body weight from established norms."

Among Singapore's teenage population, about 1,600 boys and 1,200 girls out of every 10,000 are considered obese. On the other extreme, the Eating Disorder Clinic at the Institute of Mental Health sees an average of 30 new patients suffering from anorexia nervosa a year.

Dr Sheila Loh, consultant obstetrician and gynaecologist at Raffles Hospital, said: "In both males and females, severe obesity has also been linked to other health problems, such as musculoskeletal and cardiac problems, which in turn reduce libido and cause coital difficulties."

According to Dr Fong Yan Kit, consultant urologist at Raffles Hospital, increased body fat also affects the hormonal balance between testosterone and estradiol, which affects the quality of sperm.

On the other end of the spectrum, having a low body weight is also detrimental to fertility, as thin women with a BMI of less than 18 become estrogen-deficient, resulting in irregular or absent menstrual cycles that affect their reproductive cycle.

Dr Choey advised keeping one's weight within the ideal range — not less than 95 per cent or more than 120 per cent of the recommended BMI — in order to improve fertility. - TODAY/ra