Expectant mothers' lifestyles and environment could have a 'major impact' on their sons' sperm counts, a study has found.

A review of existing research by Professor Richard Sharpe of the Queen's Medical Research Institute in Edinburgh has shown that several lifestyle and environmental factors, including smoking, obesity and exposure to chemicals and pollutants, could cause damage to both prenatal and adult testes.

Professor Sharpe also found that damage occurring during pregnancy could be irreversible. This is because exposure to toxins in the womb cuts sertoli cell numbers - cells needed to 'nurse' young sperm to maturity during a man's adult life. But he found no evidence that environmental chemicals like pesticides, food additives or persistent organic pollutants, such as dichlorodiphenyltrichloroethane (DDT), affected sperm counts in most adult men. 

A study of the after-effects of a major chemical leak from an Italian factory showed no significant effect of sperm count of men who were adults at the time. But men who were in utero had low sperm counts in adulthood.

'There is a widespread belief that environmental pesticide exposures can adversely affect sperm production in men at large. However, this seems largely untenable', he wrote, according to the Daily Mail. 'Overall, the present view is that there is no firm evidence that exposure of adult men to common environmental chemicals has any major impact on fertility'.

Dr Allan Pacey, a Sheffield University expert in male fertility, said: 'This review reminds us that the sperm production capacity of men is probably established quite early in life and perhaps even before they are born. This highlights the importance of women having healthy pregnancies and not exposing their baby to harmful chemicals, such as cigarette smoke'.