They say that the uniquely human habit allows a bug that is dangerous in pregnancy to be passed from man to woman to give her time to build up immunity.

Cytomegalovirus, which lurks in saliva, normally causes no problems. But it can be extremely dangerous if caught while pregnant and can kill unborn babies or cause birth defects.

These can include problems ranging from deafness to cerebral palsy.

Kissing the same person for about six months provides optimum protection, he added.