Statistical analysis

First, participants (n = 751, mean age 32.1 ± 4.4 years) were randomly divided into two groups (Group 1, n = 376; Group 2, n = 375) using a random sampling technique. We calculated descriptive statistics for the nine items of the MIBQ as shown in Table 1. Most of the items were positively skewed (skew values greater than 1) at all time points. We therefore log-transformed all MIBQ scores for the subsequent factor analysis. We conducted an exploratory factor analysis (EFA) of nine items of the MIBQ using data from Group 1. Because all factors were considered dependent upon each other, the factor solution was sought after Promax rotation, which is an oblique rotation. The number of factors was determined by scree plot [19]. To create a subscale of the MIBQ, we extracted items for each subscale if they were loaded ≥0.3 on a particular factor but <0.3 on the other factors.

Means and SDs of the MIBQ items at the four time points in Group 1

Each item is scored on a four-point Likert scale ranging from 0 to 3. Total scores can range from 0 to 27

MIBQ Mother-Infant Bonding Questionnaire, M mean, SD standard deviation, T1 early pregnancy before week 25, T2 late pregnancy around week 36, T3 5 days after delivery, T4 1 month after delivery

Second, the factor structure derived from the EFA was confirmed by a confirmatory factor analysis (CFA) in Group 2. The fit of each model with the data was examined in terms of chi-squared (CMIN), degree of freedom (df), comparative fit index (CFI), and root mean square error of approximation (RMSEA).

According to conventional criteria, CMIN/df < 2, CFI > 0.97, and RMSEA < 0.05 indicate a good fit, while CMIN/df < 3, CFI > 0.95, and RMSEA < 0.08 indicate an acceptable fit [20]. The Akaike Information Criterion (AIC) was used to compare different models; a model with an AIC score at least two points lower is regarded as a better model.

Cronbach’s alpha for the two hypothesized subscales was calculated to examine the internal reliability of the MIBQ.

Previous studies reported that maternal depressive mood associated with bonding disturbances [21–23]. In addition, in previous studies, EPDS was used as a way to assess construct validity for other instruments to assess bonding disturbances [14]. In accordance with previous research, we constructed a correlation matrix with the EPDS. This analysis provided evidence of construct validity.

All statistical analyses were conducted using the SPSS version 22.0 and Amos 21.0 (IBM Japan, Tokyo, Japan).