Small baby boys may be more at risk of infertility in later life, research suggests.
Scientists from Aarhus University in Denmark looked at over 10,000 men and women born between 1984 and 1987.
In 2017, the men who were small for their gestational age were 55% more likely to be diagnosed as infertile or seeking fertility treatment. The same was not true for the women born small.
“Among the men who were born small for gestational age, we found 8.3% had been diagnosed or were being treated for infertility by the end of 2017 compared to 5.7% of men born with the appropriate weight,” study author Anne Thorsted said.
Gestational age describes the number of weeks a foetus is in the womb, with “small” defined as being in the bottom 10% compared to other babies of the same age.
For instance, the study’s participants born at full term ranged from 5.5-to-9.9lbs (2.5-to-4.5kg).
Those delivered under 6.6lbs (3kg) therefore weighed less than 90% of their counterparts and were defined as small for their gestational age.
Past research links restricted growth in the womb to a higher risk of penis and testicular issues, like hypospadias - when the opening of the urethra is not at the tip of the penis, cryptorchidism - undescended testicles, and testicular cancer.
With studies few and far between - and results mixed - the scientists analysed birth records for information on gestational weight.
Decades later, they used the National Patient and In Vitro Fertilisation registries to gauge infertility rates.
Infertility affects around 12.5% of couples, with male issues responsible in around a third of cases, the scientists wrote in the journal Human Reproduction.
When they excluded men with hypospadias and cryptorchidism, the link between a small gestational age and infertility weakened.
“This may indicate 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,” Ms Thorsted said.
Hypospadias affects between one in 125-to-300 baby boys in US and “other western countries”, Hypospadias UK statistics show.
Around one in 25 boys born in the UK have undescended testicles, according to the NHS. In the US, rates vary from 1%-to-3% among full-term babies and up to 30% in those born premature.
When it comes to gestational age and infertility, other factors may also be at play.
“A suboptimal growth environment for the foetus, for whatever reason, could itself be detrimental to the development of sperm production and reproductive organs,” Ms Thorsted said.
“It could also be speculated the mother's health and lifestyle during pregnancy could affect both foetal growth and the development of reproductive functions.
“We know already if the mother smokes, this can have an impact on the foetus. It may well be cryptorchidism, hypospadias and infertility have common origins in foetal life.”
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The results revealed mothers of the low birth-weight babies were the biggest smokers and drinkers, the BBC reported.
“Our results show sometimes we must look at the very early life to find explanations of health problems that occur later in life,” Ms Thorsted said.
The scientists note the men in their study had an average age of 32 and could therefore still go on to father children. It may be worth repeating the study in around 10 years’ time, they added.
Allan Pacey, professor of andrology at the University of Sheffield, told the BBC: “It's important to note this observation does not demonstrate cause and effect, although it does fit the theory one of the most critical parts of a man's life with regard to his fertility actually occurs before he was ever born.
“This is a bit hard to get one's head around as a concept, but there is a lot of evidence emerging to support the idea, to which this new study now forms an important part.”