Hereditas 86: 45-50 (1977)

Down syndrome: Cytogenetical epidemiology MARGARETA MIKKELSEN John F. Kennedy Institute GlostruplCopenhu~en.Denmurk

MIKKELSEN,M. 1977. Down syndrome: Cytogenetical epidemiology. Lund. Sweden. ISSN 0018-0661. Received February 10, 1977

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Hereditas 86: 45-50.

The incidence of Down syndrome throughout a period of 12 years (1960 to 1971) was unchanged about 1.15 per 1000 births in parts of greater Copenhagen. 235 liveborn infants with Down syndrome were ascertained in this period in a population of I . ? million with a total of 204,771 births. The maternal age distribution changed considerably to younger maternal ages during the same period. The unchanged incidence may he explained partly by better ascertainment, hut exogenous factors may also play a role. Thus, a more successful treatment of threatening abortions and sick infants with Down syndrome may tend to increase the incidence. Also hormonal factors and factors influencing paternal non-disjunction which plays a significant role and satellite association patterns may he considered. Murguretu Mikkelsen. The John F. Kennedy Institute, DK-2600 Glostrup, Denmark

Down syndrome is one of the first recognized and best studied of the autosomal syndromes in man. It is also the most common autosomal aberration. A well established factor in Down syndrome is the maternal age effect. Since Down syndrome appears far more often in children born to older mothers than in those born to young mothers, the incidence of Down syndrome should have been falling considerably over the last 30 years, during which a remarkable reduction in maternal age at childbirth has occurred in most societies. In England, RICHARDS (1967) found a moderate increase of patients born to young mothers but a very great reduction in the number of patients born to older mothers. In 1969 a decrease in number of patients, in good agreement with the expected numbers, was observed by COLLMANN and STOLLER in Australia. In Canada, however, UCHIDA(1970) found only a slight fluctuation in the mean maternal age of mongols and a decline in incidence of Down syndrome patients was not observed in the Manitoba study. UCHIDAsuggested that exogenous factors might be involved in Down syndrome. Her observation coincides with our impression that most of the patients, the parents of whom sought advice, were born to young mothers generally below the age of 30. Therefore we decided to obtain

incidence figures in Denmark in a period when there occurred a considerable change in the age distribution of childbearing mothers and to find out if the expected fall in incidence had occurred. We chose the Copenhagen metropolitan area including the southern suburbs where the youngest maternal age population was found. This area is also served by the Center 1 of the Danish Government’s Service for the Mentally Retarded. This center has a very high standard of diagnosis, easy access to cytogenetic services and a good medical service for the population. About 1.2 million people live in this area which represents about one quarter of the Danish population. The period 1960 to 1971 was chosen. In these 12 years a considerable drop in older mothers having babies had occurred, partly because of a change in the Danish abortion legislation in 1968, giving free abortion to mothers over the age of 38. In the period 1960 to 1971, 204,771 live births were recorded in the area, 235 liveborn patients with Down syndrome being observed. The records of all patients diagnosed o r suspected of Down syndrome were examined. Both living and deceased patients were included. Data were obtained from the Service for the Mentally Retarded. Referral to the Service is compulsory by law. Data were also obtained from death certificates, records from paediatric departments,

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maternal age in this study. The inherited translocations were all ascertained through a child with Down syndrome. Mosaics were found in 2.3"". This is similar to previously reported figures. Two double aneuploidies and one double aneuploidy Age 1960 62 1963 -65 1966-68 1969 71 mosaic were observed. This is much higher than would be expected if the two events were indepenNormd populutr1 )n dent. A similar high number of double aneuploidies 7.3 13.3 12.5 15-19 13.7 was observed by HAMERTON et al. (1965). It is most 42.3 33.8 39.9 43. I 20 24 likely that a tendency to non-disjunction exists and 34.2 26.8 29.3 29.1 25 29 that the lower number of double aneuploidies in 10.2 12.0 14.5 13.4 30- 34 5.5 3.8 3.3 7.4 35 -39 some studies might reflect a higher infant mortality 1.4 1 .0 1 .0 0.9 40 44 in these cases. They may also show u p more readily 0.0 0.1 0.1 0. I 45 now when the maternal-age independent cases are 100.0 100.0 100.0 100.0 Total more frequent. As control, a normal material was collected for the DOMn sndi"ome same period and the same area. The normal material pc~pulutron 10.2 11.8 12.7 4.4 15 19 had to be sampled in two different ways, as prior to 20- 24 27.0 35.6 15.2 22. I 1968 registration was regional and after 1968 it 20.6 20.0 22.0 13.2 25 29 became centralized. For the period 1960 to 1967, 30-34 18.6 19.1 17.5 20.0 birth certificates for all infants born on the 10th of 13.3 11.1 13.2 35 39 17.0 11.1 6.7 40- 44 17.0 19.1 every month were collected for the area from the -. 1 .5 45Regional Archives and the University Hospital. For 100.0 100.0 100.0 100.0 Total the period 1968 to 1971 the normal material consists of the birth certificates for every 50th child born in the area and recorded by the Danish Health Authorities. The total control sample comprises 6053 births. obstetrical departments, maternity clinics, midwife notifications, the cytogenetic central register and the The total distribution of Down syndrome cases and central register of the Government's Service for the the calculated age distribution in percent for the Mentally Retarded. All the information was cross- normal populations is shown in Table 1. In the period 1960 to 1962, 8.97; of the control checked. All living patients were examined chromosomally. group mothers were 35 years or more, while in the In 177 cases a chromosome analysis was carried out period 1969 to 1971, 4.2% were in this age group. (75'?;;). In 5 cases the parents refused chromosome The mean age was constant at about 25 years. Of the analysis of their child and in 53 cases the patients Down syndrome population, 34% were born to died before chromosome analysis (23:

Down syndrome: cytogenetical epidemiology.

Hereditas 86: 45-50 (1977) Down syndrome: Cytogenetical epidemiology MARGARETA MIKKELSEN John F. Kennedy Institute GlostruplCopenhu~en.Denmurk MIKKE...
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