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Boys born small at heightened risk of infertility as adults

Mother’s health and lifestyle during pregnancy may be important factors in restricting foetal growth and reproductive capacity, suggest researchers

Caroline White

Friday, 13 December 2019

Boys who are born small for their gestational age may be at heightened risk of fertility problems as adults, finds research* published today in the journal Human Reproduction.

Previous research suggests that restricted foetal growth is linked to a two to three times increased risk of penile and testicular problems in boys, particularly, hypospadias, cryptorchidism, and testicular cancer, all of which are themselves  linked to fertility problems.

But it’s not clear what impact birth weight might have on the risk of infertility.

Danish researchers tracked the health and fertility, using national registry data of 5594 men and 5342 women born between 1984 and 1987 until the end of 2017, when they had reached an average age of 32.

They found that men who had been born small for gestational age were 55% more likely to have infertility problems as adults than men born within the appropriate weight range for gestational age. No such link emerged between gestational age and infertility in women.

Lead author, Anne Thorsted, formerly of Aarhus University, Denmark, said: “Among the men who were born small for gestational age, we found that 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.”

Small for gestational age is defined as a birth weight in the bottom 10% according to each week of gestation compared to other children of the same gestational age.

The weight range for babies born at full-term (40 weeks) was between approximately 2500-4500 g (5.5-9.9 Ibs) in this study, so full-term babies born weighing less than 3000 g were defined as being small for gestational age and weighed less than 90% of other children of the same gestational age.

When the researchers excluded men born with hypospadias or cryptorchidism, the link between being born small for gestational age and infertility weakened. They found a statistically non-significant 37% increased risk of infertility among the remaining men, with 7% of men born small for gestational age infertile compared to 5.6% of men with an appropriate weight at birth.

“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,” commented Ms Thorsted.

But it wasn’t yet clear what the potential mechanisms could be for the observed link between birth weight and infertility.

“A suboptimal growth environment for the foetus, for whatever reason, could itself be detrimental to the development of sperm production and reproductive organs,” she explained.

The mother’s health and lifestyle during pregnancy might affect both foetal growth and the development of reproductive capacity, she suggested.

“We know already that if the mother smokes, this can have an impact on the foetus. It may well be that cryptorchidism, hypospadias and infertility have common origins in foetal life,” she said.

The researchers acknowledge that the participants hadn’t reached the end of their reproductive life by 2017 and that different methods were used for estimating gestational age.

About 12.5% of couples are unable to conceive after a year or more. Male infertility is responsible for about a third of cases, female infertility is responsible for another third while the remaining third is due either to problems in both or to unknown causes.

 *Thorsted A, et al. Birth weight for gestational age and the risk of infertility: a Danish cohort study. Human Reproduction doi:10.1093/humrep/dez232.

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