Background: Antenatal depression is one of the most common complications among women. However, few policy-related studies have examined this problem. This study aimed to determine the prevalence of and risk factors for antenatal depression among all pregnant women first attending the obstetrics clinic in a comprehensive teaching hospital.Methods: From June to December 2019, 5780 pregnant women completed online mental health questionnaires, and data from 5728 of the women were analyzed. The women were categorized into two groups according to the presence or absence of depression. Depression was assessed by the PHQ-9, and the cutoff point for depression was 10. Anxiety and somatic symptoms were measured by the GAD-7 and PHQ-15, respectively. Univariate analysis and binary logistic regression analysis were used to determine the association among antenatal depression, anxiety, somatic symptoms and participants’ characteristics.Results: The prevalence of antenatal depression was 16.3%. Having a low level of education (P=0.012), being a rural resident (P=0.008), having an early pregnancy (P<0.0001), having a higher GAD-7 score (P<0.0001) and having a higher PHQ-15 score (P<0.0001) were risk factors for antenatal depression among women.Conclusions: One in six women was depressed throughout the pregnancy. Online mental health assessment is an effective and convenient screening method that should be widely used for pregnant women in obstetric clinics, and special attention should be paid to some risk factors (i.e., early pregnancy, anxiety symptoms, somatic symptoms, low levels of education, rural residence).