First-year postpartum depression is a common mental health problem among first-time mothers. A younger age of pregnancy often compounds the challenge due to underlying factors such as poverty and limited educational achievement. This study aimed to examine the minimal number of interpersonal supporters during pregnancy associated with lower levels of postpartum depressive symptoms among first-time mothers.

We obtained data from the population-based Mother–Infant/Newborn Tokyo Cohort (MINT cohort) in four municipalities in Tokyo on 429 first-time mothers who responded to two waves of surveys (early pregnancy and one month postpartum). They completed self-report measures of interpersonal support using one item from the Social Support Questionnaire and depressive symptoms using the Edinburgh Postnatal Depression Scale. Segmented regression analyses were conducted to determine the threshold at which the strength of the association changed between the number of interpersonal supporters and postpartum depressive symptoms, with adjustment for depressive symptoms in pregnancy. This analysis was also conducted with the sample stratified into young mothers (≤ 25 years) and older mothers (≥ 26 years).

In the overall sample, postpartum depressive symptoms were found to be lower among individuals with more than 3.0 supportive individuals (prepartum). Among young mothers, this threshold was higher, with lower symptom levels observed among those with at least 5.3 supporters. Only 22.9% of young first-time mothers had this level of interpersonal support, compared to 54.8% of all first-time mothers.

Our results suggest that having four or more interpersonal supporters in early pregnancy is associated with lower levels of postpartum depressive symptoms among first-time mothers. Additionally, among young mothers, having six or more supporters was associated with lower postpartum depressive symptoms. These findings suggest that tailored strategies to increase supporters around first-time pregnant women might be beneficial depending on their age.

Little information is available on the association between gender nonconformity during adolescence and subsequent mental health. While the distress related to gender nonconformity may be socially produced rather than attributed to individual-level factors, further research is needed to better understand the role of psychosocial factors in this context.

We analyzed data from the Tokyo Teen Cohort, obtained through random sampling of adolescents born between 2002 and 2004. We used inverse probability weighting to examine the association of gender nonconformity at ages 12 and 14 as a time-varying variable with subsequent mental health at age 16, while accounting for time-fixed and time-varying confounders. Furthermore, we used a weighting approach to investigate the mediating role of modifiable psychosocial factors in this association, addressing exposure-mediator and mediator–mediator interactions.

A total of 3171 participants were analyzed. Persistent gender nonconforming behavior at ages 12 and 14 was associated with subsequent depression (β = 2.02, 95% confidence interval [CI] 0.85 to 3.19) and psychotic experiences (β = 0.33, 95% CI 0.14 to 0.52) at age 16. The results remained robust in sensitivity analyses. Approximately 30% of the association between gender nonconformity and depression was consistently mediated by a set of psychosocial factors, namely loneliness, bullying victimization, and relationships with mother, father, and friends.

Persistent gender nonconformity during adolescence is associated with subsequent mental health. Psychosocial factors play a vital mediating role in this association, highlighting the essential need for social intervention and change to reduce stigmatization and ameliorate mental health challenges.