Sailing boats fill Stockholm harbour June 28, 2014, in preparation for the annual AF Offshore Race on Sunday. Some 300 boats are expected to be at the start line on Sunday, circumnavigating the island of Gotland in the middle of the Baltic before returning to Stockholm 2-3 days later. REUTERS/Anders Wiklund/TT News Agency

Sweden has seen a rise in baby boys being born with deformed penises, a condition called hypospadias, which has stumped the nation’s scientists.

Researchers at Stockholm's Karolinska Institute who led a 40-year study into the medical condition have found that the incidence of hypospadias has increased in babies born after 1990. The number of babies born with the condition after 1990 has reportedly jumped to 8 per 1,000 babies from 4.5 per 1,000 before this period. Hypospadias is a condition in which the urethral opening is abnormally placed and this affects urination as well as the sexual function of an individual in later life.

Scientists have not found a link to the greater incidence of hypospadias with factors such a low-birth weight; being born a twin; and in-vitro fertilization, leaving scientists without viable reasons to explain the rise, The Local reported.

Anna Skarin Nordenvall, a doctor and the lead researcher in the study, told a medical journal that an unknown factor could be behind the deformities, and added that environmental chemical agents known as endocrine disruptors could be responsible for interfering with the hormonal system, and could be a reason for the rise in hypospadias.

Endocrine disruptors are chemicals -- found mostly in pesticides, plastics and various household products -- that upset a body's hormonal balance and have been linked to cancer, birth defects and brain development problems.