Thrombocytopenia in the mother and fetus is a common problem. This chapter reviews its causes, clinical significance, investigation and management. Thrombocytopenia complicating hypertensive disorders of pregnancy is responsible for about 20% of cases of maternal thrombocytopenia. The pregnancy-related causes of disseminated intravascular coagulation (DIC) include pre-eclampsia, placental abruption, amniotic fluid embolism and, rarely, retention of a dead fetus. Immune thrombocytopenia is responsible for about 4% of maternal thrombocytopenia. Most cases are due to idiopathic autoimmune thrombocytopenia, but some are drug-related and some associated with HIV infection. Neonatal thrombocytopenia has many causes, and is the most common haematological problem in the newborn infant, and probably the fetus as well. A better understanding of the risks of thrombocytopenia in pregnancy to the mother and fetus has resulted in better targeting of treatment: withholding it in low-risk situations and using intensive therapy where there is high risk.