Stress during pregnancy is a salient risk factor for adverse obstetric outcomes. Personal capital during pregnancy, defined as internal and social resources that help women cope with or decrease their exposure to stress, may reduce the risk of poor obstetric outcomes. Using data from the 2007 Los Angeles Mommy and Baby study (N = 3,353), we examined the relationships between the balance of stress and personal capital during pregnancy, or the stress-to-capital ratio (SCR), and adverse obstetric outcomes (i.e., pregnancy complications, preterm birth (PTB), low birth weight (LBW), and small for gestational age (SGA)). Women with a higher SCR (i.e., greater stress relative to personal capital during pregnancy) were significantly more likely to experience at least one pregnancy complication, PTB, and lower gestational age, but not LBW or SGA. Accounting for pregnancy complications completely mediated the association between the SCR and PTB. Our findings indicate that experiencing greater stress relative to personal capital during pregnancy is associated with an increased risk for pregnancy complications, PTB, and lower gestational age and that pregnancy complications may be a mechanism by which the SCR is related to adverse obstetric outcomes.