Swedish scientists are struggling to explain a rise in cases of hypospadias, a condition that causes baby boys to be born with deformed penises.
A 40-year study conducted by researchers at Stockholm's Karolinksa Institute found that incidences of hypospadias in babies born after 1990 jumped significantly from 4.5 boys out of every 1,000 to eight per 1,000.
The scientists said that they had taken into account factors such as low-birth weight that could be linked to the condition, but still could not explain the rise, The Local reported.
Hypospadias means the urethral opening is abnormally placed and can affect urination and sexual function later in life.
Lead researcher Dr Anna Skarin Nordenvall told Dagens Medicin newspaper that an unknown factor could be behind the deformities.
She said that scientists could not rule out the theory that chemicals - specifically environmental agents known as endocrine disruptors - could be responsible for the trend.
Endocrine disruptors, which are mostly man-made, can be found in pesticides, plastics and various household products.