"We need to look more closely at the role of environmental factors, including those that can disrupt the hormone system, and the role of genetics, lifestyle and other factors."
The study found that cryptorchidism was associated with low birth weight, preterm delivery, small gestational weight and other congenital abnormalities of the genitalia. An interesting significant risk factor was the father's weight; babies born to underweight fathers (body mass index less than 20kg/m2) were six times more likely to have undescended testes.
However, Dr Jorgensen said: "This finding should be treated with caution because the number of cases was rather low and only larger studies will be able to show whether this is a genuine association or not."
The study did not find a statistically significant difference between boys born to mothers who smoked or who worked in a potentially harmful environment where they might be exposed to heat, vibration or chemical, but the prevalence of cryptorchidism was slightly higher. It rose from 5.1% in non-smoking mothers to 7.8% in smoking mothers, and from 4.9% to 7.13% for mothers working in a potentially harmful environment.
"Although this increased risk was not statistically significant, it suggests that environmental factors could be important," said Dr Jorgensen.
 Exposure to persistent organochlorine pollutants associates with human sperm Y:X chromosome ratio. Human Reproduction. doi:10.1093/humrep/deh855
 Higher than expected prevalence of congenital cryptorchidism in Lithuania: a study of 1204 boys at birth and 1 year follow-up. Human Reproduction. doi:10.1093/humrep/deh887