This is a cross-sectional study and sub-study of the New Families (NF) research project, a prospective non-randomized controlled study. The NF research project is registered at clinicaltrial.gov (identifier: NCT04162626) and approved by the Regional Committees for medical and health research ethics in Norway (reference no: 2018/1378) and the Norwegian Centre for Research Data (project no: 735207).
The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist for cross-sectional studies  was followed for the reporting of this study.
Setting and participants
The setting of this study was clinics in the Norwegian Child Health Services, including the prenatal care provided to pregnant women by midwives. Pregnant women were screened for eligibility and invited to participate in the study by the midwife or the clinic secretary, when attending pregnancy check-ups in five districts in the city of Oslo, from October 2018 to December 2019. Pregnant women and their partners were contacted by a researcher for informed consent. Inclusion criteria were pregnant women and men with pregnant partners expecting their first child and living in one of the five city districts of Oslo. The exclusion criteria were multiparous women.
We were not allowed to collect background information on participants who were unwilling to participate. Thus, selection bias is possible if our respondents differ from non-respondents. The number of respondents eligible and willing to participate during the study period determined the sample size.
Self-reported measures were sent to the participants by mail. The consent form and all measures were available in ten languages (Norwegian, English, Arabic, Lithuanian, Pashto, Polish, Somali, Tamil, Turkish, and Urdu). All data were returned by the end of January 2020.
Standard demographic data were measured, including family income, educational level, age, nationality, and marital status. In addition, hours of sleep and previous and present mental illness were measured.
Quality of life was measured by the World Health Organization Quality of Life Questionnaire Brief version (WHOQOL-BREF) . The instrument contains 26 items, which includes one item from each of the 24 facets of WHOQOL-100 and two single items on overall QoL and general health satisfaction. The single items are examined separately, and the remaining 24 items produce the four dimensions physical health (7 items), psychological (6 items), social relationship (3 items), and environment (8 items). All items are assessed on a 5-point Likert scale (range 1–5), with various scale responses. Higher values indicate higher QoL. The domain scores, ranging from 4–20, were calculated by multiplying the mean score of each domain by four, according to the WHOQOL-BREF scoring manual . WHOQOL-BREF is validated in the general Norwegian population [29, 30]. The instrument’s psychometric properties have been reported for both pregnant women and men with pregnant partners . In our study, Cronbach’s alpha for the WHOQOL-BREF domains physical health, psychological, social relationship, and environment were, respectively, 0.80, 0.81, 0.67, and 0.78 for the pregnant women and 0.74, 0.85, 0.54, and 0.72 for the men with pregnant partners.
Selective possible predictive factors
Perception of sleep was measured by a single item: Do you feel that you get enough sleep (Yes/No)? Complications during pregnancy were also measured by a single item: Did you have any complications during your pregnancy (Yes/No)?
The Edinburgh Postnatal Depression Scale (EPDS)  was used to assess depressive symptoms. Respondents are asked about symptoms in the past seven days. The EPDS contains ten items scored on a 4-point Likert scale, ranging from 0 to 3. The total score of the scale ranges from 0–30, with higher scores indicating higher levels of depressive symptoms. A cut-off score of 10, where < 10 indicates no depressive symptoms and ≥ 10 indicates depressive symptoms. The EPDS has been validated for postnatal use in women  in the Norwegian population. Validation studies have been conducted with pregnant women [34,35,36] but not men in international studies. In our study, Cronbach’s alpha for the total scale of EPDS was 0.83 for the pregnant women and 0.74 for the men with pregnant partners.
Sample characteristics were described separately for pregnant women and men with pregnant partners using descriptive statistics. Categorical data were presented as counts and percentages, and continuous variables were described as means and standard deviation (SD). Crude comparisons of the pregnant women and men with pregnant partners on background variables were performed using t-test for continuous variables and chi-square for categorical variables. As the level of education and family income were highly correlated, we used the level of education only as a possible predictive factor in all analyses to avoid multicollinearity. The number of missing values for each item is presented.
Descriptive analyses were used to determine the WHOQOL-BREF single items and domain scores in pregnant women and men with pregnant partners, described by mean and SD. Chi-square test was used to determine the association between pregnant women and men with pregnant partners on the two WHOQOL-BREF single items, described by p-value. Independent sample t-test was used to determine the differences in WHOQOL-BREF domain scores between pregnant women and men with pregnant partners, described by mean difference, 95% confidence intervals (95% CI), and p-value.
Moderation analyses were used to explore the relationship between the four dependent variables of WHOQOL-BREF domain scores and the independent variable of perception of sleep and the possible moderating effect of depressive symptoms. Moderation analyses were performed as described by Hayes , using the PROCESS macro with model 1. The results are presented with unstandardized coefficients (B) and 95% CI. All the confidence intervals were derived using bootstrapping with 5000 repetitions. Age, pregnancy week, complications during pregnancy, and educational level were included as confounders in the model. Furthermore, multivariate linear regression was used in the sub-analysis of pregnant women to investigate if the selected predictive factors were statistically significantly associated with the WHOQOL-BREF domain scores when stratified by depressive symptoms. Educational level was treated as an ordinal variable with three levels in all regression analyses. All other variables were continuous or dichotomous. Internal consistency reliability was examined by calculating Cronbach’s alpha for all four domain scales of WHOQOL-BREF and the total scale of EPDS.
All statistical analyses were conducted in SPSS, version 28, in the secure platform of Services for Sensitive Data . The level of statistical significance was set to p < 0.05 for all analyses, and all point estimates are reported with 95% CI.