BACKGROUND:Approximately one‐fifth of women present depression during pregnancy and puerperium, and almost 13% of pregnant women experience a major depressive disorder.OBJECTIVE:The aim of this study was to identify risk factors for depression among pregnant women with a medical disorder and to evaluate the influence of depression on perinatal outcomes.METHODS:Three hundred and twenty‐six pregnant women with a medical disorder were interviewed. A semistructured interview was conducted for each participant using a questionnaire that had been developed previously. Major depression was diagnosed using the Portuguese version of the Primary Care Evaluation of Mental Disorders (PRIME‐MD). The medical records of the participants were thoroughly reviewed to evaluate the perinatal results.RESULTS:Major depressive disorder was diagnosed in 29 cases (9.0%). The prevalence of major depression was as follows: 7.1% for preeclampsia or chronic hypertension, 12.1% for cardiac disorder, 7.1% for diabetes mellitus, 6.3% for maternal anemia, 8.3% for collagenosis and 12.5% for a high risk of premature delivery. An univariate analysis showed a significant positive correlation between an average household income below minimum wage and a PRIME‐MD diagnosis of major depression. A multiple regression analysis identified unplanned pregnancy as an independent predictor of major depression (86.2% in the group with a diagnosis of major depression by PRIME‐MD vs. 68.4% in the group without major depression). A comparison between women who presented major depression and those who did not revealed no significant differences in the perinatal results (i.e., preterm delivery, birth weight and low Apgar scores).CONCLUSION:In the present study, unplanned pregnancy in women with a medical disorder was identified as a risk factor for major depression during gestation. Major depression during pregnancy in women with a medical disorder should be routinely investigated using specific methods.