Human pregnancy filtering: yesterday's news?

Forbes[1xForbes, L.S. Trends Ecol. Evol. 1997; 12: 446–450Abstract | Full Text PDF | PubMedSee all References[1]suggests that the filtering efficiency of a proposed screening mechanism for defective embryos may adaptively decrease with maternal age in humans (adaptive-filtering hypothesis). However, neither the decreasing filtering ability nor any adaptive function are supported by the evidence cited. In fact, the well-known increase in chromosomal error rates in ova with maternal age (by-product hypothesis) perfectly predicts `the rising incidence of chromosomal abnormalities in live-births, the falling incidence of normal embryos in spontaneous abortions, and the increased incidence of spontaneous abortions with maternal age' (Ref. [1xForbes, L.S. Trends Ecol. Evol. 1997; 12: 446–450Abstract | Full Text PDF | PubMedSee all References[1], p. 446). These relationships would straightforwardly follow from an increased chromosomal aberration rate, given that such aberrations cause insufficient embryonic function which lead to pregnancy termination.The crucial point for the adaptive-filtering hypothesis would be, however, that this probability would decrease with age. There is a clear prediction here: the increase of the proportion of aberrant offspring with age should be steeper for live births than for abortions. Forbes[1xForbes, L.S. Trends Ecol. Evol. 1997; 12: 446–450Abstract | Full Text PDF | PubMedSee all References[1]erroneously suggests (p. 447) that his Box 5 implies that the adaptive-filtering hypothesis predicts increased proportions of abnormal embryos in abortions: reasonable alterations of the overlap of distributions and positioning of thresholds in Box 5(b) can obviously produce opposite trends!The other relevant tests reported by Forbes are conflicting or controversial. The proportion of maternal-origin trisomies does not increase with age according to a cytogenetic study, but does so according to molecular genetic findings. And the magnitude of increase in the proportion of defective ova does not meet the increase in chomosomal aberration rates in live births. However, the crucial finding of lower aberration rates in abortions is not mentioned.Even if one had evidence for alteration of the filtering mechanism, the adaptive value would be a matter of additional enquiry. While one can always construct the appropriate fitness scenarios (and such fitness effects need not exist in today's societies), testable predictions still emerge. For instance, the costs of bearing an aberrant embryo must be higher, the younger other dependent offspring are. Likewise, the benefit of additional children clearly diminishes with family size. Hence, the proportion of aberrant live births should decrease with increased family size and increase with increased age of the previous child.As long as one cannot provide evidence for predictions that differentiate between the hypotheses, the existence of an adaptive maternal screening mechanism in human pregnancies is bound to be a mere conjecture from the zeitgeist. This conjecture is not new[2xShepard, T.H. and Fantel, A.G. Clin. Perinatol. 1979; 6: 216–243See all References[2], but the persisting lack of decisive evidence (though relatively easy to compile) reveals the ambition still to be vaulting.