Objectives: The purpose of the study was to examine the relationship between self-reported stress levels among new mothers in São Paulo, Brazil and two biomarkers of stressful experience, oxytocin (OT) and Epstein-Barr Virus antibody level (EBV-ab), with planned pregnancy hypothesized as a moderator of biological response to stressful conditions.

Methods: Sixty-three first-time mothers between the ages of 15 and 45 were recruited from neighborhoods in São Paulo, Brazil. Quantitative and qualitative data were collected longitudinally, bi-weekly between two and 12 weeks postpartum. OT level was assessed from breast milk samples and EBV-ab from blood spot samples. An Interpersonal Satisfaction scale was developed, validated, and administered, along with the Cohen perceived stress scale (PSS).

Results: In-depth interview data revealed unplanned pregnancy to be a significant stressor in the lives of first-time mothers. In linear regression, OT level was negatively associated with interpersonal satisfaction score (P = 0.022) and positively associated with PSS score (P = 0.007). When splitting the sample by planned status of the pregnancy, women with an unplanned pregnancy showed a strengthened positive association between OT level and PSS (P = 0.001; Adj R(2) = 0.44) and negative association with interpersonal satisfaction (P = 0.017; Adj R(2) = 0.15), while no associations existed for women with a planned pregnancy. EBV-ab level was not correlated or associated with stress/satisfaction measures.

Conclusion: OT is an effective biomarker in the measurement of stress in the body, and additionally reflects differential experiences with difficult interpersonal circumstances, such as unplanned pregnancy. By contrast, EBV-ab failed to reflect differences in self-reported stress levels between mothers.