"One way to further improve the estimate of the abortion rate is to account for at least some of the occult abortions that are not detected by high-sensitivity comes from studies comparing aneuploidy in very young embryos (cleavage stage and blastocysts) to conceptuses that spontaneously aborted after a pregnancy had been clinically detected. At the early embryo stages, monosomy (missing one homolog from one or more of the 23 pairs of homologous chromosomes) and trisomies (a extra chromosome at one or more of the 23 pairs of homologous chromosomes) were found to be equally common. But among conceptuses that survive to clinical detection, and then abort, essentially no autosomal monosomies were observed (<<1%). This repeated finding demonstrates that all autosomal monosomic conceptuses spontaneously abort before a pregnancy can be detected by elevated levels of hCG, i.e., prior to implantation. As a consequence, for every observed aborted conceptus that carried one or more trisomies after clinical detection, there must have been one occult aborted conceptus due to one or more monosomies. At least 50% of aborted fetuses are karyotypically abnormal and at least 60% of these carry one or more trisomies. There are therefore at least 0.5*0.6* = 0.3 occult abortions due to monosomy for every detected post-implantation abortion. In Fig. 3, I have added these additional occult abortions to the total."