The sons of women who smoke are more likely to have low sperm counts, a new study has warned.
Researchers found that men who smoke or take drugs are also more likely to suffer fertility problems.
Several studies over the last 20 years have suggested that semen quality is in decline, reflected most clearly in falling sperm counts.
The problem has been attributed to environmental factors - such as exposure to toxins - and to men smoking.
However, a new study suggests that exposure to several factors before birth, and in early life, may also lead to reduced semen quality in adulthood.
The study, found that slow foetal growth, exposure to a mother who smokes, and slow growth in childhood were all associated with a subsequent decline in sperm production.
The study, which is being reported today at the annual meeting of the European Society of Human Reproduction and Embryology (ESHRE) in London, was based on the Western Australian Pregnancy (Raine) Cohort which began in 1989 with the enrolment of 2,900 mothers-to-be.
Their babies had regular assessment before and after birth.
At the age of 20 to 22 their sons took part in a testicular assessment, which included measurement of testicular volume, analysis of semen quality, and analysis of hormone production.
Results showed that about one in six of the men tested had sperm parameters below the ‘normal’ threshold recently defined by the World Health Organization (WHO).
And more than a quarter of the subjects had sperm whose appearance did not meet the WHO's acceptable criteria.
When the results were compared with the earlier foetal growth assessments, being consistently small in the womb was associated with a significantly greater chance of having a low sperm count.
Being exposed to their mother smoking was also associated with lower sperm production.
Roger Hart, Professor of Reproductive Medicine at the University of Western Australia, says that poor foetal growth, exposure to maternal smoking, poor childhood growth patterns, increased fat deposition in adolescence, and smoking and drug use in adulthood may ultimately lead to reduced sperm production.
Professor Hart said: ‘The main message from our study is that to reach adulthood with the best possible testicular function a man should not be exposed to his mother's smoking, should have good foetal growth and, in childhood and through adolescence, should be “appropriately grown” - that is, neither underweight nor overweight, and as an adult should not smoke or take drugs.’
Professor Hart added: ‘The extent of the risk posed by environmental endocrine disrupters is still unclear, but some researchers do attribute the perceived decline in sperm counts to these chemicals within the environment.
‘We do not have any evidence to suggest such a link in our study, but we do intend to measure the foetal exposure to endocrine disrupting chemicals from maternal blood that was stored in 1990, prior to the study recruits' birth, and to relate these chemical exposures to the men's semen counts in 2012-3.’