Participants
The present study utilized data collected from the Panel Study on Korean Children (PSKC). The PSKC is an ongoing longitudinal panel study conducted by the Korea Institute of Child Care and Education since 2008. The participants in PSKC were invited by stratified multistage sampling using resident registration data to represent all nationwide household populations. A total of 2,150 children born between 2008 and 2009 in Korea were enrolled in the study from the fetal period and evaluated prospectively for breastfeeding and cognitive development at 5.5 (T1), 14.1 (T2), 26.2 (T3), 38.7 (T4), and 99.2 (T9) months of age. Because of the challenges of longitudinal cohort studies, there was some missing data for the follow-up assessments. In our study, we analyzed data collected from 1,752 children whose assessments of breastfeeding and K-ASQ at T3 (26.2 months) were present.
Measurements
Demographic variables including the child’s sex, age, gestational period, birth weight, parents’ education level, and household income were assessed by paper and pencil interviews and computer-assisted personal interviews. Breastfeeding data from T1 (5.5 months) to T4 (38.7 months) was prospectively collected by computer-assisted personal interviews.
Early development
To assess early cognitive development at T1 (5.5 months), T2 (14.1 months), and T3 (26.2 months), the participants were assessed using the Korean Ages and Stages Questionnaires (K-ASQ) and the Korean version of Denver II, which are widely used screening tools for early development. K-ASQ is a screening tool for the developmental progress of infants and toddlers as rated by parents [12]. The K-ASQ comprised 30 items rated on three-point Likert scales under the following five subdomains: communication, gross motor, fine motor, problem-solving, and personal–social [13]. Scores that were two standard deviations below the average in each subdomain were coded as atypical. Denver II is another screening tool for early development with a validated Korean version [14, 15]. Denver II codes the development of children to the dichotomous outcomes of “normal” or “suspicious” based on the assessment scores. Denver II comprised four subdomains: personal–social, fine motor/adaptive, language, and gross motor. K-ASQ and Denver II could not be included because of copyright; however, additional information regarding both tools can be obtained from the Panel Study on Korean Children website [16].
Cognitive function in middle childhood and school age
Language abilities at T4 (38.7 months) were assessed by the Receptive and Expressive Vocabulary Test (REVT), which is comprised of 185 Korean vocabulary items and two subscales of receptive and expressive language tests [6]. The REVT results were coded to the percentile scores of 1 (<10%) to 11 (100%) as a continuous variable and “Normal or mildly delayed” and “Markedly delayed” as binary categorical variables. The cognitive function of school-age children at T9 (99.2 months) was assessed in terms of intelligence and academic performance. The intelligence of children was assessed using the multifactorial intelligence test (M-FIT). The M-FIT is comprised of six subdomains (vocabulary, language inference, schematization, calculation, spatial perception, and reasoning), each with 20 item tests. The scores of M-FIT are presented with the T-score and percentile score (0–100) based on normative data. Our analysis used the T-score, which is a norm-referenced standardized score with a mean of 50 points and standard deviation of 10 points. The T-score of ≤37 on each subscale was labeled as “delayed,” and the participants with the “delayed” scores on at least one subscale were categorized as “delayed” for the categorical analysis.
Statistical analysis
Although the criterion for breastfeeding duration to group participants vary widely in previous studies, many studies included “never,” “1 month,” “3 months,” “6 months,” and “12 months” as the duration criteria [6]. In our study, participants were grouped by the following breastfeeding durations according to previous studies [9, 18]: “never,” “up to 1 month,” “1–3 months,” “3–6 months,” “6–12 months,” “12–18 months” and “over 18 months.” We used logistic regression to investigate the odds ratio for delayed development of the early period (T1, 5.5 months to T3, 26.2 months) assessed by Denver II and K-ASQ. To compare the outcomes of K-ASQ as continuous variables, language ability at T4 (38.7 months), and intelligence and academic function at T9 (99.2 months) among the groups of breastfeeding duration, analysis of variance (ANOVA) and analysis of covariance (ANCOVA) were utilized. In all analyses, the adjusted model included the children’s sex, age, gestational age, birth weight, parents’ education level, and household income as covariates. To adjust for multiple comparisons included in our analysis, we performed the Benjamini–Hochberg test with a false discovery rate threshold of 0.05 for the crude and adjusted models, respectively [19]. Statistical analyses were conducted using the software package SPSS 25.0 for Windows (IBM Co., Armonk, NY, USA).
Ethics
We provided parents with information on the purpose and procedure of the study and written informed consent was obtained from parents before enrollment. This study was performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. The study protocol was approved by the Institutional Review Board of the Korean Institute of Child Care of Education (KICCEIRB-2016-07).