Study Design and Data Sources
We conducted a register-based cohort study using nationwide registers. Data linkages were achieved via the personal identity number, which is assigned to all Danish residents at birth or upon taking up residency21. Register keepers at Statistics Denmark carried out data collection and register linkage with all data having been anonymized before the researchers gained access to the data.
From The National Patient Register22 information about parental diagnoses, birth-related variables of the child and child in- and outpatient hospital contacts as well as emergency department (ER) contacts were obtained. The diagnoses were encoded using the International Classification of Diseases, 10th Revision (ICD-10). The Danish Health Service Register for Primary Care23 provided information on contacts to general practitioners (GPs), private psychiatrists and psychologists. Information on reimbursed drug prescriptions were obtained from the Danish National Prescription Registry, which uses the Anatomical Therapeutic Chemical (ATC) Classification System24. Data on parity and gestational age were accessed through the Danish Medical Birth Register25 and on deaths through the Danish Death Register. Information on parental highest completed education were extracted from the Population Education Registry26. Family income were obtained from the Income Statistics Register27. Date of birth, parental civil status, child gender, country of origin and information on whether the parents lived together with each other as well as with the child were obtained from the Danish Civil Registration System21. The included Danish registers are in general considered to be of high quality with complete long-term follow-up28.
Settings, Study Population And Follow-up
The Danish healthcare system is characterized by free access. Services at general practice (GP) and public hospitals are funded by the Danish tax system and free of charge. Acute medical assistance is only delivered by public services. GP serves as gatekeeper for the hospitals, and contacts to the hospitals require a referral from the GP.
All live-born Danish children born from January 1 2000 to December 31 2016 were identified and followed the first 12 months of life. Children were excluded if they died or emigrated during the first year of life or did not live with any of the parents at the time of birth.
Parental Mental Health
Parental mental health was categorized in three (no mental health condition, minor and moderate-severe) using The National Patient Register, The Danish Health Service Register for Primary Care and the Danish National Prescription Registry. In Denmark, minor mental health conditions are treated by GPs or psychologists in the primary healthcare sector and are thus not registered in The National Patient Register29. GPs serve as gatekeepers in the primary healthcare sector and refer more severe mental health conditions to assessment and treatment in the secondary healthcare sector (in- and outpatient wards at the hospitals or private psychiatrists). Diagnoses are not registered in The Danish Health Service Register for Primary Care; therefore, minor mental health conditions need to be identified otherwise. Talk therapy and performed psychometric tests at GP as well as contacts to psychologist are registered in The Danish Health Service Register for Primary Care and were used to identify minor mental health conditions. Furthermore, reimbursed prescriptions of antidepressant and anxiolytic medication were used. Other psychotropic medication, for example antipsychotics, were not included due to broader indications, for example nausea or analgesics.
Moderate to severe mental health conditions were identified as a registered psychiatric diagnosis (ICD-10 F00-99) at psychiatric hospital or contact to private psychiatrist. All the mental health condition indicators were measured within five years before the birth of the child. A reference group was defined as no mental health-related contacts to GP, no contacts to psychologist or private psychiatrist, no reimbursed prescriptions of antidepressants or anxiolytic medication and no registered psychiatric diagnosis within five years before the birth of the child (see Box 1).
These two groups, minor (group 1) respectively moderate-severe (group 2) mental health conditions, and the unexposed reference group generated a matrix of exposures for the child due to the combinations of the mother respectively the father in the different groups.
Box 1. Criteria for exposure. Consensus definition of groups of mental conditions. | |
Exposure | Specification At least one criterion fulfilled | Further criteria All criteria fulfilled | Healthcare sector and registry |
Group 1 Minor mental health conditions | Medication - At least 2 prescriptions of: - antidepressant medicine (ATC N06AB, N06AX) - anxiolytic (benzodiazepines: ATC N03AE, N05BA, N05CD, N05CF) Services at general practice - At least two ‘talk therapy’ - At least two psychometric tests Other services - At least one contact to private psychologist | No contacts to psychiatric hospital and no psychiatric hospital diagnoses No records of contact to private psychiatrist | Primary healthcare sector The Danish Health Service Register for Primary Care Danish National Prescription Registry |
Group 2 Moderate to severe mental health conditions | - Any registered psychiatric diagnosis (ICD-10 F00-99) at psychiatric hospital - Mental health conditions treated at private psychiatrists (including child and adolescent psychiatrists) | | Secondary healthcare sector The National Patient Register The Danish Health Service Register for Primary Care |
Reference group No mental health condition | None of above - No registrations of psychiatric diagnoses and no mental health condition-related contacts to GP, psychologist or private psychiatrist and - No prescriptions of antidepressants or anxiolytic drugs | | |
Exposure was measured in a period of 5 years before the birth of the child | |
Outcomes
All healthcare service contacts within the first year of life were identified for every child. Vaccinations and routine childcare visits were not included in this study. A contact to GP daytime or out-of-hour service was defined as either consultation, telephone contact or visit. Due to changes in the delivery of out-of-hour service in the Capital Region in 2014, children from this region were excluded from the out-of-hour analyses from 2014–2016. An inpatient contact was defined as any admission to hospital, and an outpatient hospital contact as any registered contact to an outpatient unit. Due to changes in the registration of neonatal outpatient contacts during the study period, only outpatient contacts after the neonatal period (the first 28 days of life) were included. Any registered contact to the emergency department was defined as an ER contact.
Covariates
Covariates, that potentially could confound the association between parental mental health conditions and child use of healthcare services, were identified by reviewing the literature. A directed acyclic graph30,31 was constructed to evaluate the identified covariates (see supplements). All covariates were extracted at the time of birth of the child, except family income, which was extracted the calendar year before.
Based on the International Standard Classification of Education (ISCED 2011)32, parental education was grouped according to highest completed education in three: 1: Early childhood education, primary and lower secondary education (ISCED levels 0–2); 2: General upper secondary education, high school programs, vocational upper secondary education, vocational training and education (ISCED 3–4); and 3: Short-, medium-length or long-length higher education, first-, second- or third-cycle programs, tertiary education, bachelor or equivalent, Master’s or equivalent, Doctoral, PhD programs or equivalent (5–8).
Family income was defined as the equivalated disposable income for the family that take into account the number of children and adults in the household. We adjusted for family income, parental age at time of birth of the child and calendar year using restricted cubic splines with three knots33.
Family type was classified as either living with two biological or adoptive parents or not.
Small-for-gestational-age (SGA) weas defined as a birthweight for gestational age under the 2.3-percentile and large-for-gestational-age (LGA) as a birthweight for gestational age over the 97.7-percentile based on the national guidelines34,35. Parity was grouped in two according to whether the child was the first child of the mother or not.
Statistics
Baseline information was analyzed and reported as percentages based on maternal respectively paternal mental health condition. In the following analyses, the maternal and paternal groups were combined and in total, eight exposure groups and one reference group were generated.
The median number of events for each outcome variable was calculated, and for each exposure group with an outcome above the median the total number and percentages was calculated. Poisson regression analyses36, crude and adjusted, were applied to the number of outcome events for each child during follow-up to estimate the incidence rate ratio (IRR) of experiencing the outcome.
Siblings appear in the cohort and should preferably be treated in the analyses as dependent. However, a mixed effects model was not possible to apply to such a large dataset. Instead, we made sensitivity analyses including only first born children.
Non-fatal birth defects, prematurity and other significant conditions in early life might necessitate a higher number of healthcare contacts the first year of life. To examine whether such conditions affected the estimates, a sensitivity analyses excluding children admitted for at least one week during the first month of life.
Data was analyzed using Stata SE 15.1.