Ethanolamine kinase catalyzes the first step in the CDP-ethanolamine pathway for the formation of the major membrane phospholipid phosphatidylethanolamine (PtdEtn). In this work, the predicted Etnk2 cDNA was established as a soluble protein with ethanolamine-specific kinase activity that was most highly expressed in liver. Mice with an inactivated Etnk2 gene were derived, and its absence reduced the rate of PtdEtn synthesis from exogenous ethanolamine in hepatocytes. PtdEtn is a major precursor to phosphatidylcholine in liver; however, Etnk2(-/-) mice did not have reduced amounts of either PtdEtn or phosphatidylcholine or an altered phospholipid molecular species distribution. The knock-out animals were able to adapt to a choline-deficient diet. The Etnk2(-/-) mice exhibited a maternal-specific intrauterine growth retardation phenotype that resulted in a 33% reduction in litter size and frequent perinatal death. Histological analysis of pregnant Etnk2(-/-) females showed that fetal development failed at the late stage of pregnancy in a significant percentage of embryos because of the appearance of extensive placental thrombosis. These results illustrate a non-redundant role for EtnK2 expression in regulating placental hemostasis.