Underweight baby boys may have raised risk of infertility as adults

Boys born underweight for the amount of time they spent in the womb may have a higher risk of infertility in adulthood, a Danish cohort study has reported.

It found that men born 'small for their gestational age' had a 55 percent greater risk of infertility as adults, when compared with men born within the normal weight range for gestational age. Babies defined as small for gestational age were in the lowest 10 percent body weight compared to other infants of the same gestational age.

'We found that 8.3 percent had been diagnosed or were being treated for infertility by the end of 2017 compared to 5.7 percent of men born with the appropriate weight,' said lead author Anne Thorsted at Aarhus University in Denmark. The researchers analysed 5594 men born between 1984 and 1987 in two Danish cities, until they were aged 32 on average.

She added: 'Our results show that sometimes we must look at the very early life to find explanations of health problems that occur later in life.'

But the link weakened when excluding men born with certain genital issues, such as hypospadias (when the urethra's opening is on the underside of the penis) and cryptorchidism (undescended testicles). Past studies have found low birth weight linked to genital issues, and genital issues linked to fertility problems, such as lower sperm counts.

'The genital malformations may account for part of the observed association in the main analysis, but this must be explored further,' Ms Thorsted told The Guardian.

The participants' mothers had also completed questionnaires during pregnancy looking at factors that could affect the results, including age, health and lifestyle. However, it is still unclear what mechanism is behind the link between low birth weight and infertility.

'A suboptimal growth environment for the fetus, for whatever reason, could itself be detrimental to the development of sperm production and reproductive organs,' said Ms Thorsted. 'It could also be speculated that the mother's health and lifestyle during pregnancy could affect both fetal growth and the development of reproductive functions; for instance, we know already that if the mother smokes, this can have an impact on the fetus. It may well be that cryptorchidism, hypospadias and infertility have common origins in fetal life.'

The researchers acknowledged that participants were in their early 30s and still had a long reproductive life ahead of them - therefore it could be interesting to analyse the data in another ten years' time.

Professor Christopher Barratt at the University of Dundee said: 'It all points to the idea that part of male infertility is a consequence of in utero development and birth.' He added: 'What comes first and what causes what, we are probably less clear about.'

Professor Richard Sharpe at the University of Edinburgh said: 'In utero factors are important to male fertility, but it is unlikely underweight babies are behind the large declines in sperm count across western populations.'

The study, published in Human Reproduction, also looked at 5342 women but found no link between low birth weight for gestational age and infertility among women.