Objective: The prevalence rate of obsessive-compulsive disorder (OCD) was found to be higher in women during pregnancy and puerperium than that estimated in the general population. Additionally, the symptomatology of OCD shows several variations during the lifetime of women. Objective of the current study was to examine the clinical characteristics and comorbidity with other anxiety and mood disorders of pregnancy-onset obsessive-compulsive disorder (POCD) and to investigate factors related to POCD. Method: The study sample was composed of three groups. The first group (POCD group) included 20 consecutive pregnant women meeting the criteria of the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) for OCD, showing an onset of OCD during their current pregnancy. The second group (non-OCD group) consisted of 207 consecutive pregnant women without any mood or anxiety disorders to assess factors associated with POCD. In addition, the study included a control group to compare the symptoms of POCD and non-pregnancy-onset OCD. The control group was composed of 40 nonpregnant female outpatients with OCD. Type and severity of obsessive-compulsive symptoms were assessed with the Yale-Brown Obsessive-Compulsive Scale (YBOCS). OCD and other anxiety or mood disorders were determined by means of the Structured Clinical Interview for DSM-IV (SCID-I). Comorbid axis II disorders were diagnosed with the Structured Clinical Interview for DSM-III-R Personality Disorders (SCID-II). Results: The mean onset time of OCD in the POCD group was 13.3±6.35 (week5-28) gestational weeks. OCD occurred during the first trimester of pregnancy in 9 women (45%), during the second trimester in 10 (50%) women, and during the third trimester in 1 (5%) woman. The most common obsessions were contamination (n=16, 80%) and symmetry/exactness (n=6, 30%), and the most common compulsions were cleaning/ washing (n=16, 80%) and checking (n=12, 60%) in POCD. Thirteen (65%) of the pregnant OCD patients met the criteria for a mood or anxiety disorder. Generalized anxiety disorder was the most frequently diagnosed axis I disorder (40%, n=8). The independent factors associated with POCD were cigarette smoking (p=0.002), the existence of an anxiety disorder at onset of pregnancy (p=0.000), and obsessive-compulsive personality disorder (p=0.003). Conclusion: The present study suggests that POCD presents similar clinical characteristics with nonpregnancy-onset OCD. Mood or anxiety disorder comorbidity is observed in more than half of the women with POCD. Additionally, pregnant women who have at least one of 3 factors (cigarette smoking, the existence of an anxiety disorder at onset of pregnancy, and obsessive-compulsive personality disorder) seem to be at risk for POCD.