Pregnancy-induced hypertension

Cases of pregnancy-induced hypertension were identified from international Classification of Diseases (ICD-9) codes. Four separate case groups were created based on these codes: eclampsia (ICD-9 code 642.6, n = 154), severe preeclampsia (ICD-9 code 642.5, n = 1,180), mild preeclampsia (ICD-9 code 642.4, n = 5,468), and gesta- tional hypertension (ICD-9 code 642.3, n = 8,943). All cases identified during the study period were included. Controls (n = 47,237) were defined as women who did not have any of the above diagnoses. Controls were randomly sampled from each year of the study and were frequency matched to the cases by year of birth. There were 215,897 total possible controls from which this sample was drawn.

Explain what type of study design is this? State your reason.