Background: Ulcerative Colitis (UC) has a marked influence on the lifestyle of patients, and its effects on pregnancy and childbirth can especially become a problem for women in their child-bearing years. Various studies have suggested that it is desirable for pregnant women with UC to give birth while remaining in a state of remission. The present study evaluated pregnant women with UC attending our hospital who became pregnant during remission, in order to examine the factors that contributed to recurrence of UC during pregnancy. Methods: We investigated 40 pregnant patients in remission (44 cases) attending our hospital between January 2008 and June 2016 who had remained in remission for one year prior to pregnancy. After becoming pregnant while in remission, patients who stayed remission until delivery were classified into the ongoing remission group (35 cases) and patients with recurrence during pregnancy were classified into the recurrence group (9 cases). Items examined: Clinical characteristics, the CAI and whether or not patients continued treatment during pregnancy were examined and compared between the two groups. The reasons for discontinuation of treatment were also investigated. Results: There were significant differences between the two groups with respect to the age of becoming pregnant, the CAI in the first, second, third trimester, and whether oral treatment was continued (continuation of treatment [yes:no] 30:5 in the ongoing remission group vs. 5:4 in the recurrence group). Regarding the discontinuation of oral treatment, two patients in the ongoing remission group and one patient in the recurrence group discontinued it on their own judgment, while two patients in the recurrence group discontinued it due to hyperemesis. Conclusions: The present study revealed that factors influencing the recurrence of UC during pregnancy were the age of becoming pregnant and the continuation of oral treatment. As expected, discontinuing oral treatment was a factor that contributed to recurrence. However, the reasons for discontinuing treatment during pregnancy differed from those for non-pregnant women. Some patients discontinued treatment on their own judgment because they were concerned about adverse effects on the fetus, while others had difficulty with continuing treatment due to hyperemesis. With regard to the effects of medications on the fetus, medical staff should provide an explanation about the safety of treatment and should be aware that patients may have various concerns about drug therapy. If patients have difficulty continuing oral treatment due to severe hyperemesis, administration of local therapy should be considered. During pregnancy, it is important to continue treatment for UC so that patients can give birth while remaining in remission.