Men's sperm count could be reduced by exposure to chemicals in the environment, according to research.
A rise in IVF treatments could be down to the effect of chemicals such as cosmetics, detergents and pollutants.
Researchers from the University of Glasgow, in collaboration with academics in Edinburgh, the James Hutton Institute, the University of Aberdeen and INRA in France, think the chemicals affect a certain subset of men.
The researchers looked at the testicles of sheep that had been exposed to a range of chemicals that humans encounter in everyday life. They found abnormalities that could result in low sperm counts in the testicles of 42% of the animals.
The changes were not the same in all affected individuals and they were not obvious from the size of the testicles or from the concentration of male hormones in the blood.
The year-long experiment saw 12 sheep grazed on land that had solid human waste applied to it. The animals' mothers were also grazed there, meaning the sheep were exposed throughout their life cycle.
The treatment was at a low concentration and in line with regulations.
Twelve control sheep also took part in the experiment.
The animals were then euthanised and their testicles examined.
Three sheep had smaller testes than normal, with a reduction in the number of sperm-producing germ cells found in the tissue of the testes. Two other sheep's testes looked normal but also showed the same reduced germ cells.
Professor Neil Evans, from the University of Glasgow, said it was unclear why five of the sheep were affected and the others were not. He said it could be because of genetic factors or because of the way they had been exposed to the chemicals.
He said: "These findings emphasise that even when the concentration of single chemicals in the environment may be very low, it is hard to predict what the health effects are when an individual is exposed to a mixture of chemicals.
"This finding adds to previous work conducted by this group that has shown effects on male and female reproductive organs, and some of the systems within the body that regulate reproduction, in young animals born to mothers exposed to this environmentally relevant mixture of chemicals."
The results could suggest that a rise in the need for in-vitro fertilisation in humans is due to exposure to chemicals in our environment, the team said.
Figures released last year showed a 5.9% rise in IVF treatments between 2009 and 2010, with 45,264 women being treated with either IVF or intra-cytoplasmic sperm injection (ICSI), according to the Human Fertilisation and Embryology Authority.
The research, published in the International Journal of Andrology, was funded by University of Aberdeen-coordinated grants from the Wellcome Trust and the European Community's Seventh Framework Programme.