Socio-occupational class, region of birth and maternal age: influence on time to detection of cryptorchidism (undescended testes): a Danish nationwide register study

Each boy was linked to his parents using the CPR-number in the Fertility Database[19], which enabled identification of parental employment as the basis for socioeconomic position[20, 21]. In Denmark, all persons are classified annually according to occupation by Statistics Denmark from 16 years of age onwards. Specifically, socioeconomic position was defined by the NYSTGR classification, a specialised version of the Danish version the International Standard Classification of Occupations (ISCO) 1968 version, used from 1980 to 1995 by Statistics Denmark[22, 23]. Socioeconomic position was defined the year before birth of the child, to avoid change in maternal occupational status due to maternity leave and change of occupational status, e.g. to become an (unemployed) house wife. The groups were defined and ranked as: white collar group 1 (senior salaried staff including managers etc. with more than 20 employees), white collar group 2 (leading salaried positions requiring higher academic education, i.e. generally more than 4 years beyond high school), white collar group 3 (other salaried staff, e.g. office and service workers, nurses, laboratory technicians, pre-school teachers), skilled workers, and unskilled workers. Since both maternal and paternal occupational levels contribute to the socioeconomic environment of the family, one common variable was constructed, using the highest occupational level from either the father or the mother as default[21]. In cases where the father could not be identified, maternal social class was used. Self-employed and individuals outside the workforce constitute two very heterogeneous groups that cannot readily be ranked with respect to socioeconomic position; the latter contains students, unemployed with various skilled background, house wives, and recipients of social security. We therefore included only children with at least one employed parent in the study population. If one parent was self-employed or outside the workforce, socioeconomic status of the employed (not self-employed) parent was used. Accordingly, children for which both parents were self-employed, outside the workforce, or self-employed combined with assisting spouse, were excluded from the study population. In total 730 out of 12,006 boys with cryptorchidism were excluded due to the above criteria. Table 1 presents the social status of the father and mother of the excluded boys.

TTD was assessed by region of birth, i.e. one of 15 Danish counties in existence between 1970 and 2006: Copenhagen, Frederiksborg, Roskilde, Western Zealand, Storstrøm, Bornholm, Funen, Southern Jutland, Ribe, Vejle, Ringkøbing, Aarhus, Viborg, Northern Jutland and the combined municipalities of Copenhagen and Frederiksberg. In essence, the region of birth corresponded to maternal region of living at the time of parturition. This variable was available for more than 99% of all children.

TTD was assessed by maternal age, divided into four categories (<25, 25–29, 30–34, ≥35 years).