Men who were born underweight at greater risk of fertility problems, study suggests

Men who were born underweight are more likely to struggle to have children of their own, a new study suggests.

Researchers in Denmark looked at 5594 men and 5342 women born between 1984 and 1987 and followed them through to adulthood until the end of 2017 when they were aged 32 on average.

They found that men who had been born under 6.6lbs had a  55 per cent increased risk of infertility as adults compared to men born within the appropriate weight range for gestational age.

Although in a normal population around 5.7 per cent of men would be expected to suffer fertility problems, for those born underweight the figure rose to 8.3 per cent.

Yet no link between weight and infertility in women.

Speaking about what may cause the effect, lead researcher Anne Thorsted, of the Department of Public Health at Aarhus University, said: “A suboptimal growth environment for the foetus, for whatever reason, could itself be detrimental to the development of sperm production and reproductive organs.

“It could also be speculated that the mother's health and lifestyle during pregnancy could affect both foetal growth and the development of reproductive functions. For instance, we know already that if the mother smokes, this can have an impact on the foetus.

“Our results show that sometimes we must look at the very early life to find explanations of health problems that occur later in life.”

The weight range for babies born at full term (40 weeks) was between approximately s 5.5-9.9 lbs (2500-4500 grams) so full-term babies born weighing less than 6.6lbs (3000 grams) were defined as being small for gestational age and weighed less than 90 per cent of other children of the same gestational age.

Previous research has suggested a link between poor growth in the womb and a two to three times increased risk of problems with the sexual organs or reproductive tract in boys, such as hypospadias when the urethra occurs in the wrong place or cryptorchidism when the testes are undescended. Such conditions are also linked to fertility problems.

In the current study, when the researchers excluded men born with hypospadias or cryptorchidism, the link between being born small for gestational age and infertility weakened.

Ms Thorsted said: “This may indicate that part of the association between gestational weight and infertility is mediated by the effects of hypospadias and cryptorchidism, which is known to be related to later risk of infertility.”

The researchers obtained information on birth weight and gestational age from birth records, and retrieved information on infertility diagnoses and fertility treatments from the Danish National Patient Registry and the Danish In Vitro Fertilisation registry.

The mothers of all the participants had completed a questionnaire during pregnancy that asked about factors that could affect the results, such as age, body mass index (BMI), smoking drinking and sociodemographic status so that other factors could be excluded.

The researchers said the participants had not reached the end of their reproductive life by 2017 and would like to see what the situation was in another ten years' time.

The research was published in the journal Human Reproduction.