In preeclampsia, plasma antithrombin activity is decreased, which leads to exacerbation of the disorder. We previously showed that long-term magnesium sulfate (MgSO4) administration prolonged the pregnancy period and may be able to improve pregnancy outcomes for patients with severe preeclampsia. The present study aimed to investigate the changes in plasma antithrombin activity during long-term MgSO4 administration for patients without severe hypertension. This multicenter retrospective study included patients with preeclampsia and superimposed preeclampsia without severe hypertension at diagnosis. The participants were divided into two groups: MgSO4 nontreatment group (three institutions) and MgSO4 treatment group (one institution). Antithrombin activity from time of diagnosis to delivery were compared between the two groups. In the MgSO4 nontreatment group (n = 16), antithrombin activity prior to delivery was significantly lower than at time of diagnosis (p = 0.015). In three cases, antithrombin activity was less than 60%. On the other hand, in the MgSO4 treatment group (n = 34), antithrombin activity did not change until just before delivery (p = 0.74). There were no cases in which antithrombin activity was decreased below 60%. Long-term MgSO4 administration for preeclampsia without severe hypertension may prevent a decrease in antithrombin activity and improve the disease state of preeclampsia.