Aging gametes in relation to incidence, gender, and twinning in Down syndrome.

Sexual practices have been implicated in the causation of Down syndrome (DS). Two mechanisms have been hypothesized. The theory of the fertilisation of an overripe ovum due to a delay in fertilization, and more recently the aging of the sperm. The orthodox Jewish population have very specific, strict practices concerning menstruation, which provide a unique model for the study of births in relation to time of conception. Data on the incidence of DS in the Jewish population of Jerusalem between 1982 and 1988 were analyzed. Among a comparable group of mothers under age 37 years and for whom amniocentesis was not available, it was found that the incidence of DS was significantly higher in the religious than in the nonreligious population (P less than 0.001). The rate of twinning in the DS population was significantly higher than in the general population (P less than 0.015), as was the ratio of males to females, 62% and 38%, respectively compared with 51% males and 49% females in the newborn population (P less than 0.001). The association of these 3 factors in relation to the aging gamete of either maternal or paternal origin is discussed.