"Pregnancy rates fall and miscarriage rates rise when the father is over 35," says Dr Belloc. "This is the first time a strong effect of the father on reproduction has been shown."
Dr Belloc and colleagues followed 21,239 intrauterine inseminations (IUI), in which sperm is washed or spun to separate it from the seminal fluid and then placed directly into the womb. (Unwashed semen would cause cramping of the womb and expel the sperm, because of the prostaglandins in seminal fluid.) A total of 12,236 couples were followed between 2002 and 2006; the partner's sperm was used in every case, and in most cases the man was being treated for infertility.
The sperm was checked before insemination for count, movement and shape, and pregnancy, miscarriage and birth rates were recorded, with tests to separate female from male factors.
As expected, the mother's age was linked with lower pregnancy rates: 8.9% in women over 35 against 14.5% in younger women, and miscarriage rates were also affected by the mother's age. But the man's age also mattered: men over 35 had a negative effect on pregnancy rates, and miscarriage rates rose when the father was over 35. This may be due to damage to sperm DNA, which increases with age, although the evidence for this is not yet conclusive.
The researchers say the strong link between the father's age and IUI results should be taken into account by doctors and patients, and that IVF or ICSI may be suggested for infertile couples where the man is over 35. In IVF, the outer layer of the egg (zona pellucida) may act as a barrier to sperm with damaged DNA, and in ICSI the best sperm can be chosen. These methods don't guarantee success, but they may help couples with an older male partner conceive more quickly and lower the risk of miscarriage. The study was presented to the European Society of Human Reproduction and Embryology.
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