During pregnancy, women experience various emotional changes. Previous research has investigated positive emotion influenced childbirth outcomes positively. This study explored the extent of positive emotion which pregnant women aware of their own pregnancy life subjectively and variables related to the changes thereof. This was an adjunct study of the Japan Environment and Children's Study which is a nationwide epidemiological study to identify the impact of environmental factors on health and development of children in Miyagi Prefecture. Data were collected at 12 weeks (T1) and 24-28 weeks (T2) of gestation. Participants were 3,513 pregnant women. Women's degree of positive emotion was assessed through a numeric rating scale. We also measured related variables, including obstetric factors, physical health status, mental health status, and social situation, using the 8-Item Short Form Health Survey, Kessler-6, and family APGAR score, along with demographic data. Analyses included correlations among variables related to the change in positive emotion. The extent of positive emotion was significantly related with health-related quality of life and satisfaction with family relationships during pregnancy. The women whose positive emotion decreased from T1 to T2 tended to have lower morning sickness symptoms, be unemployed, lower physical and mental health status, and a higher risk of psychological distress in middle pregnancy. Importantly, even discomfort like morning sickness, physical symptoms that indicate being pregnant, might make women to confirm pregnancy. Moreover, employment and satisfaction with family relationships are especially crucial for pregnant women to spend comfortable maternity life.