Objective: The principal aims of this study were to examine the current prevalence rate, clinical characteristics, and related factors of obsessive-compulsive disorder (OCD) in pregnant women during the third trimester of pregnancy. Method: The study data were gathered from 434 consecutive women in the third trimester of pregnancy who presented to the obstetric outpatient clinics of 2 university research centers and from 58 consecutive nonpregnant women with diagnosed with OCD who presented to the psychiatric outpatient clinics of the same centers. Obsessive-compulsive disorder was diagnosed by means of the Structured Clinical Interview for the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition. The Yale-Brown Obsessive-Compulsive Scale was used to determine the severity and types of obsessions and compulsions. Results: The prevalence rate of OCD was found to be 3.5% among the women in the third trimester of pregnancy. Two (0.5%) women reported that OCD developed during the second trimester (16th and 24th gestational weeks) of pregnancy. The most common obsessions were contamination (80.0%) and symmetry/exactness (60.0%), whereas the most common compulsions were cleaning/washing (86.7%) and checking (60.0%). Women with pregnancy-onset OCD and some women with previous diagnoses of OCD had obsessions and compulsions with themes focused on the fetus or newborn. Pregnant women with OCD had higher frequencies of family history of OCD compared with women without this disorder. Age, educational level, employment status, number of gestations and live births, history of abortion, frequency of primigravida, and the existence of gestational complications were unrelated to OCD in the pregnant women. Pregnant and nonpregnant women with OCD had similar characteristics of obsessive-compulsive symptoms. Conclusion: Our study suggests that OCD is present relatively frequently among pregnant women during the third trimester of pregnancy, and it has similar clinical features during gestation and nongestation. © 2007 Elsevier Inc. All rights reserved.