Background: The present study was aimed at investigating etiologies, complications, outcomes of pregnancies complicated by moderate and severe thrombocytopenia. Methods: A retrospective analysis of 26 pregnant women with moderate to severe thrombocytopenia (platelet count <100 x 109/L) who delivered between August 1, 2006 and December 31, 2010. Results: During the study period, total 4822 deliveries were performed in Kangnam sacred heart hospital and the incidence rate of moderate to severe thrombocytopenia was 0.54%. The main causes of thrombocytopenia were immune thrombocytopenic purpura (ITP) and gestational thrombocytopenia (GT), which occupied 42.3% and 38.5% respectively. Other etiologies were preeclampsia (2 patients), vitamin B12 deficiency (2 patiens), and early liver cirrhosis (1 patient). Two patients with preeclampsia were diagnosed with myelodysplastic syndrome (MDS) and thrombotic thrombocytopenic purpura (TTP) respectively after delivery. Comparing with ITP and GT patients, median platelet counts at diagnosis were significantly different (44 x 109/L vs. 91 x 109/L respectively). However, median platelet counts at delivery day were similar (93 x 109/L vs. 99 x 109/L). Eleven patients were received platelet transfusion (median value 16 unit) during peripartum period. Two patients with preeclampsia and two patients with ITP had preterm births. An adverse perinatal outcome was rare and only one fetal intrauterine growth retardation was observed. Conclusions: Moderate to severe thrombocytopenia in pregnancy was rare in clinical practice. The most common causes of thrombocytopenia in pregnancy were ITP and GT. The peripartum outcomes of ITP and GT were favorable. However special attention should be given to patients with preeclampsia and other rarer causes.