Cytogenetic analysis of 1000 spontaneous abortions showed 463 to have an abnormal chromosome constitution. The proportion of chromosome abnormalities varied with the gestational age of the abortus and the type of tissue cultured but was not significantly different among the five racial groups represented in the study population. It was suggested that differences in the rate of chromosome abnormalities among cytogenetic studies of spontaneous abortions were the result of methodological differences in sample selection rather than real biological variation among study populations. The only factor found to be unequivocally associated with the aetiology of chromosome abnormalities in spontaneous abortions was increasing maternal age in trisomies.

One hundred and six triploids were ascertained during a study of 1500 consecutive spontaneous abortions. The mechanism of origin of the additional haploid complement was investigated by comparing parental and foetal cytogenetic heteromorphisms and a histopathological examination of each triploid was done in a subsequent blind study. The mechanism of origin of the additional haploid complement was found to be highly correlated with the development of partial hydatidiform mole and with gestational age. All 51 paternally derived triploids in which a pathologic diagnosis could be made were partial moles, whereas only 3 of 15 maternally derived triploids on which a diagnosis could be made were molar. The mean gestational age of the paternally derived triploids was 122 days while that of the maternally derived triploids was only 74 days. It was suggested that the development of partial mole was primarily associated with the presence of two paternal haploid chromosome complements, the association with relatively long gestational ages being a secondary one consequent upon retention of the molar placentae for many weeks after foetal demise.

Cytogenetic results on 234 spontaneous abortions showed 109 (46.6%) to have a chromosome abnormality. The results were analysed separately for the four major ethnic groups of which the population was composed and no differences in the proportion or types of chromosome abnormalities seen. The results of this survey were found to be very similar to other major published surveys and it therefore appears, on present evidence, that neither geographic location nor racial group affects the frequency of chromosome abnormalities seen among human spontaneous abortions. The sex ratio among the chromosomally normal and abnormal abortuses and the effect of parental and gestational age were discussed.

Summary. The results of a 5 year prospective cytogenetic and epidemiologic investigation of complete and partial hydatidiform mole are described. A total of 40 complete and 88 partial moles were identified by pathologic criteria in a population of 1602 spontaneous abortions. Cytogenetic observations confirmed the close correlation between chromosome constitution and histopathologic type, virtually all the complete moles were paternal parthenogenones and virtually all the partial moles were triploid. The distributions of maternal age, race, place of birth and socioeconomic status were the same in women with partial moles and in the control population of spontaneous abortions. However, the incidence of complete moles was significantly increased in women aged <20 years and also in Filipino women, these two effects being independent of one another.

A cytogenetic survey of 475 patients in an institution for the mentally retarded is reported. The chromosomes of all patients were studied using both a non‐banding and a G‐banding technique in order to estimate the relative efficiency of the two techniques in detecting structural rearrangements of the chromosomes. A total of 57 patients was found to have a chromosome abnormality, including five with a balanced structural rearrangement. The contribution of chromosome aberrations to the etiology of mental retardation is discussed with special emphasis on the contribution of balanced structural rearrangements.