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Fathers of non-identical twins have better sperm

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    Fathers of non-identical twins have better sperm 13:28 20 February 2007 NewScientist.com news service Michael Day Why older mums expect more twins 04 March
    Message 1 of 1 , Feb 20, 2007
      Fathers of non-identical twins have better sperm
      • 13:28 20 February 2007
      • NewScientist.com news service
      • Michael Day

      Fathers of non-identical twins have better quality sperm than other men, according to a study by Danish fertility specialists. The observation suggests that twinning rates could provide a useful measure of male fertility in a given population.

      Earlier research had revealed that men who took longer to father a child were less likely to have non-identical (dizygotic) twins. And some research had hinted that non-identical twins were less common in populations with low male fertility. But there was little or no research on semen quality in fathers of twins, until now

      A team at the University Hospital of Copenhagen compared sperm quality in

      37 men who had fathered non-identical twins with a control group of 349 men with normal, healthy sperm. On average, they found that fathers of twins had more normal and motile, or mobile, sperm than the control group. Their sperm counts were also higher, although not significantly.

      "Our results support the idea that changes in semen quality may influence the non-identical twinning rates," says team leader Camilla Asklund. Indeed, the researchers conclude that the prevalence of non-identical twinning in a population could prove a useful measure of male fertility.

      Genetic differences

      However, Lynn Fraser, a sperm expert at Kings College London, UK, suggests that genetic differences between different populations may be at least as important in determining the prevalence of non-identical twins.

      "We know that in the Yoruba tribe in Nigeria, for instance, the rate of dizygotic twinning is 5.2%, which is very high, and this must be down to genetic differences that mean the women there are more likely to shed two eggs at once," Fraser says. "And let's not forget, if the woman doesn't release two eggs, it's not possible to have dizygotic twins in the first place."

      The Danish researchers concede that the chances of having non-identical twins is generally higher in older women, as the probability of releasing two eggs in one cycle rises with maternal age. Historically, about one birth in 80 gives rise to twins (1.3%). This rate has increased in some Western countries - particularly the US - in recent years due to the rise in IVF treatments.

      However, if maternal factors are considered equal among fathers of twins and fathers of singletons, differences in the fertilisation of the two eggs released are probably related to semen quality, they say.

      As part of the study, the researchers also tested the sperm quality of 15 men who had fathered identical (monozygotic) twins. Unexpectedly, they found that these men also had higher than average sperm quality. They were unable to explain this observation and say the finding might be an anomaly that occurred to due to the small sample size.

      Journal reference: Human Reproduction (vol 22, p 751)


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