Data on psychiatric disorders came from the Danish Psychiatric Central Register, which holds information on all admissions to psychiatric hospitals in Denmark since 1969. Furthermore, it contains records of psychiatric contacts at outpatient clinics since 1995. At present, the register holds information on more than 725 000 persons and 3.25 million contacts. From 1969 to 1993, the diagnostic system used in the registries was the Danish adaption of the International Classification of Diseases, Eighth Revision (ICD-8)13 and ICD-10 from 1994 and onward.14 The study population and their parents were linked to the register to obtain information on mental illness to study psychiatric disorders in both the probands and presence or absence of psychiatric disorders in their families. Women and their parents were classified as having a psychiatric disorder if they had records of either inpatient or outpatient contacts at a psychiatric hospital with any type of mental illness (main diagnoses only). For the women in the study, diagnoses were grouped as follows: bipolar affective disorders (ICD-8 codes 296.19, 296.39, and 298.19; ICD-10 codes F30 and F31), unipolar affective disorders (ICD-8 codes 296.09, 296.29, 296.99, 298.09, 300.49, and 300.1x; ICD-10 codes F32, F33, F34.1, F38.8, and F39.0), schizophrenia-like disorders (ICD-8 codes 295.xx, 297.xx, 298.39, and 301.89; ICD-10 code F2x.xx), adjustment disorders (ICD-8 codes 307.xx and 308.4x; ICD-10 code F43.xx), personality disorders (ICD-8 code 301.xx minus 301.83; ICD-10 codes F60 and F61), puerperal disorders not elsewhere classified (ICD-8 code 294.49; ICD-10 code F53.xx), and other disorders (remaining diagnoses). Parents of the women in the study were categorized by their history of psychiatric disorders in the following way: history of psychiatric contact with bipolar affective disorder (ICD-8 codes 296.19, 296.39, and 298.19; ICD-10 codes F30 and F31), history of psychiatric contact with any type of mental illness excluding bipolar disorder (remaining diagnoses), and no history of psychiatric contacts.