Early Fetal Wastage
The majority of human conceptions end in early fetal wastage, often associated with aneuploidy (multiple chromosomal abnormalities). Many of these events occur so early that they are sometimes mischaracterized as "delayed menstrual periods". This phenomenon has not been observed in great apes in captivity (including some populations in which female cycles were monitored carefully during breeding programs). Thus, it may not occur in great apes - at least not at the frequency observed in humans. One possible explanation is that the unpredictable timing and frequency of copulation in humans leads to fertilization of sub-optimal ova.
A very large number of human conceptuses are spontaneously aborted (~ 30%) and the majority of these spontaneous abortions result from chromosomal errors (trisomies, monosomy-X, triploidy)1. Some of these abortions occur so early as to be ascertained only by delayed or excessive menstruation. Reviews of material from zoos and primate research centers show that such abortions are either very uncommon or that they do not occur.
There have been a few spontaneous chromosomal errors reported in apes: Trisomy 22 (same as human 21) in a chimpanzee, an orangutan and perhaps in a gorilla. Trisomy 18 in bonobo; deletion of portion of chromosome 3q in gorilla. Triploidy in a fetal chimpanzee2.
The reason why so few spontaneous abortions are observed in nonhuman primate colonies is not understood. Also, only few spontaneous chromosomal errors have been described in nonhuman primates, e.g. monosomy X in rhesus.
Partial hydatidiform mole in a pregnant chimpanzee (Pan troglodytes), , Zoo Biology, Volume 12, p.299–305, (1993)
Fréquence des aberrations chromosomiques dans les avortements spontanés humains, , C. R. Acad. Sci., Volume 269, p.283–288, (1969)