Previous population studies

In the literature on PWS, prevalence has been variously quoted as ‘about 1 in 25,000 live births’, ‘between one in 25,000 and one in 10,000 live born children’, ‘[estimates] vary 6-fold from 1 in 5,000 to 10,000; 1 in 10,000; 1 in 15,000; 1 in 25,000; to 1 in 10,000 to 30,000’. These estimates are confusing because some authors use the term prevalence where others use birth incidence. We use birth incidence to refer to the number of live births of babies with PWS and prevalence to refer to the number of people with PWS of all ages in the population. Only two estimates appear to be based on epidemiological data, those of Akefeldt et al. and Burd et al. (see Chapter 1). In the North Dakota study (Burd et al.) the authors used similar informants to those contacted in the Cambridge study. The ascertainment methods used in the North Dakota study identified eight males, eight females and one person whose gender was not given, with an age range from 9 to 30 years. At that time the population of North Dakota for that age range was 263 444, giving a prevalence rate of 1:16 062, equivalent to 1:38 395 in the entire population. No figures were given for the number of people with a genetic diagnosis, and the diagnostic assessment was based, in at least some cases, on a one-page questionnaire pictorially demonstrating the signs of PWS to aid identification. Birth incidence was not estimated.