Study Population
The Norwegian Mother and Child Cohort Study
The present study is based on the Norwegian Mother and Child Cohort Study (MoBa) (17). The MoBa study is a prospective population-based cohort study conducted by the Norwegian Institute of Public Health, with the purpose of gaining knowledge of the causes of diseases by assessing possible associations between selected exposures before and after birth and given outcomes among children. From 1999 to 2008, pregnant women from all over Norway were invited to participate in conjunction with their routine ultrasound scan appointment in around week 17 of pregnancy. About 41% of the invited women consented to participate. There were no exclusion criteria, but the mothers had to be able to read Norwegian to be included. The cohort includes more than 114,000 children, 95,000 mothers, and each woman could participate with more than one pregnancy. Follow-up was conducted by questionnaires at regular intervals (at gestational week 15 and 30, and when the child was 6 months and 18 months, and 3, 5, and 8 years), and by linkage to national health registries (17). Written informed consent was obtained from all MoBa participants upon recruitment. The study was approved by the Regional Committee for Medical Research Ethics in South-Eastern Norway. Participants did not receive any financial compensation.
For the present study, we have analyzed data on 86,724 children. Twins (n = 1,480) and triplets (n = 14) were excluded. Data was collected at four time points, when the child was 6 months (Questionnaire 4), 18 months (Questionnaire 5), 3 years (Questionnaire 6) and 5 years (Questionnaire 5).
Measures
Demographic Variables
Demographic variables included child gender, gestational age, maternal age, maternal education level, civil and work status.
Emotional and Behavioral Problems (CBCL)
The Child Behavior Checklist (CBCL) (18) is constructed to cover a range of emotional, social, and behavioral problems. The CBCL/1.5-5 consists of 99 items describing behavior exhibited by the child during the preceding two months. The CBCL/1.5-5 has five subscales (Emotionally Reactive, Anxious/Depressed, Somatic Complaints, Attention Problems, and Aggressive Behavior). These subscales can be combined into two aggregated scales measuring internalizing behavior problems (Anxious/Depressed, Somatic Complaints,) and externalizing behavior problems (Attention Problems, Aggressive Behavior). Each item rates on a scale from 0 (not true), 1 (somewhat or sometimes true), or 2 (very true or often true). The CBCL has been proven to be a valid and reliable tool (19). Space restrictions in the questionnaires led to an abbreviated CBCL/1.5-5 scale with 30 CBCL items being included in the MoBa study. A previous publication (20) from the MoBa study showed that the items assessing externalizing problems in the abbreviated version of the CBCL were representative of the full externalizing CBCL scale, with a correlation of 0.92. In the present study, the CBCL was collected at 18 months, 3 years and 5 years.
The Ages and Stages Questionnaire (ASQ II)
The ASQ (21), is a parent-completed developmental screening instrument, consisting of 19 age-specific questionnaires intended for use from the age of 4 months to 60 months The ASQ covers five developmental areas: communication, gross motor, fine motor, problem solving, and personal-social development. Parents were asked to evaluate whether their child had achieved a number of specific skills and rate how true statements were for their child, as yes, sometimes, or not yet. The ASQ has shown good test–retest reliability (94%), and concurrent validity when compared to standardized tests (76–88%) (21). The Norwegian version of the ASQ has also shown good validity (22). In the MoBa study, an abbreviated ASQ scale was used, including a 13-item scale at 18 months, a 10-item scale at 3 years and a 12-item scale at 5 years, respectively. In the present study, the ASQ was collected at 18 months, 3 years and 5 years.
Sleep-related Variables
Sleep duration at 6 months was assessed by the question, ‘How many hours does your child sleep per day’. The response categories were 10 hours or less, 11 to 12 hours, 13 to 14 hours, and 15 hours or more. The two latter categories were combined to form the reference category based on the most frequent sleep durations at 6 months. Nighttime awakenings were assessed by a single question, ‘How often does your child usually wake during the night?’. Response categories were 3 or more times every night, once or twice every night, a few times a week, and seldom or never. The two latter categories were combined to form the reference category. The last analyzed sleep-related item was ‘Easy to put to bed and falls asleep quickly’. The response categories were a seven-point scale from ‘do not agree at all’ to ‘strongly agree’. The categories were combined to form the reference categories agree and do not agree.
Colic
Colic was assessed by the parent reporting on a question when her child was 6 months: ‘Has the child had colic?’.
The response categories were yes or no. Parents thus answered yes if they thought their child had had colic, and no other diagnostic criteria were needed.
Statistical Analyses
Continuous variables were described with mean and standard deviation (SD), categorical variables with counts and percentages. Both main outcomes were assessed using a questionnaire administered at four time points. The number of included items varied among assessment points so we used z-scores to be able to assess differences between groups of children both at given time points and across the whole follow-up. The z-scores were constructed as follows: at each assessment point the dataset was divided into two groups based on the exposure variable (e.g. having colic or not, having a sleep problem or not). Those who did not report such an event (no colic, no sleep problems) served as the normal population for a given outcome (event). Thus z-scores equal to zero represent the mean for the normal population, scores over zero indicate higher values of the outcome compared to the norm, and scores below zero indicate values lower than 50% of the norm.
To model changes at given time points and across the follow-up trajectories, we used linear mixed models for repeated measures with an unstructured covariance matrix to account for dependencies within the included individuals (children), as the same child was assessed at several time points. The results are expressed as estimates of beta (B) with 95% confidence intervals (CI). P-values < 0.05 were considered statistically significant. As the study is considered exploratory, no correction for multiple testing was done. All analyses were performed using SPSS, version 24.