Sharav [19911 reports an age-independent increased incidence, a high sex ratio, and twinning rate in children with Down syndrome (DS) born in Jewish orthodox versus nonreligious hospitals in Jerusalem. This is of great interest because of the possible linkage with aging gametes, but it contrasts with the lower incidence of abortions [Brunner, 19411, infant mortality [C.B.S., 1936-19381, and central nervous system defects [Naggan and MacMahon, 19671 mentioned previously in Jewish (orthodox) groups living in the Diaspora. Ritual cleansing 7 days after the last trace of menstruation (the mikve) has, indeed, been interpreted to promote optimum conceptions, characterised by a rather synchronous presence of fresh gametes at the fertilization site, comparable with midcycle conceptions [Jongbloet and van Erkelens-Zwets, 19781.In order to explain her findings, Sharav [1991] invokes either delayed fertilization by aged sperm (because of abstinence from intercourse by the husband for the 2-week period prior to ovulation), or delayed fertilization of the egg (i.e., postovulatory overripeness ovopathy; PoOO) because of 3 other possibilities: (1) prolonged menstruation, (2) too short a follicular phase, and (3) a voluntary delay in going to the mikve. However, about 95% of the DS children are shown to be maternal in origin (Antonarakis et al, 1991) and neither aging of sperm nor PoOO is able to reconcile the presence of 2 different maternal 21 chromosomes evident in most DS individuals. The related nondisjunction, indeed, occurs during the first meiotic division prior to ovulation. This is even more pertinent for a double ovulation leading to dizygotic twin pairs, one ofwhich affected by DS. For the explanation of both these phenomena a preovulatory mechanism has to be assumed. I propose delayed ovulation or preovulatory overripeness ovopathy (PrOO), which is in line with the seasonality of maternal meiotic I nondisjunctions in DS individuals [Jongbloet et al., 1982; Jongbloet and Vrieze, 19851. If so, the incidence of delayed ovulation should be higher in contemporary Jerusalem orthodox women. I wonder whether Sharav's [1991] orthodox population contains a relatively large number of less affluent and more deprived individuals gathered from various geographic origins and characterised by being less fecund, more seasonally dependent, and, hence, more prone to cycle disturbances [Jongbloet, 19911. This subfecundity concept explains Sharav's excess of sibships that included both a twin pair and a DS infant. In addition, it is in line with the more pronounced seasonality in DS births according to the number of years of maternal education in another West Jerusalem DS study by Harlap 119741.
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