A new research made by a team of Danish fertility specialists at the University Hospital of Copenhagen discovered that men fathering non-identical (dizygotic or from two different eggs) twins have better quality sperm than those who don't.

The investigation points that twinning levels could represent a useful clue in assessing male fertility in a given population.

Previous investigations showed that men who took longer to have a child also presented a lower possibility of producing non-identical twins and the percentage of non-identical twins was very low in populations with low male fertility.

This research is the first on this field investigating the semen quality in fathers of twins.

The research team made a comparison between the sperm of 37 men with dizygotic twins, 15 fathers of identical (monozygotic or produced by the partition of a single-fecundated egg) twins and a control group of 349 healthy men with normal sperm that fathered singletons.

Scientists discovered that fathers of twins possessed more normal and mobile sperm cells than their counterparts and the total number of sperm cells was slightly higher.

"Our results support the idea that changes in semen quality may influence the non-identical twinning rates," said team leader Camilla Asklund.

Other researchers believe that differences in the gene pool amongst populations have their importance in triggering the incidence 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," said Lynn Fraser, sperm expert from Kings College London, UK.

"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 team agrees that older mothers present a higher chance of delivering non-identical twins as their probability of releasing two eggs in one cycle is increased.

In the general population, 1.3 % of the births are twin births but this percentage has risen in some Western countries (like US) due to "In Vitro Fertilization" treatments.

"However, if maternal factors are considered equal among fathers of twins and fathers of singletons, differences in the fertilization of the two eggs released are probably related to semen quality," said Asklund.

The research pointed that also the fathers of identical twins had higher sperm quality than the average, but due to the small number of subjects, these data have no statistical power.