Obstetricians frequently diagnose thrombocytopenia in pregnant women because platelet counts are included with automated complete blood cell counts obtained during routine prenatal screening (). Although most U.S. health care providers are trained using U.S. Conventional Units, most scientists, journals, and countries use Système International (SI) units. The laboratory results reported in U.S. Conventional Units can be converted to SI Units or vice versa by using a conversion factor. Given the conversion factor is 1.0, when converting from 10/μL to 10/L the platelet "count" does not seemingly change. Thrombocytopenia, defined as a platelet count of less than 150 × 10/L, is common and occurs in 7-12% of pregnancies at the time of delivery (). Thrombocytopenia can result from a variety of physiologic or pathologic conditions, several of which are unique to pregnancy. Some causes of thrombocytopenia are serious medical disorders that have the potential for maternal and fetal morbidity. In contrast, other conditions, such as gestational thrombocytopenia, are benign and pose no maternal or fetal risks. Because of the increased recognition of maternal and fetal thrombocytopenia, there are numerous controversies about obstetric management of this condition. Clinicians must weigh the risks of maternal and fetal bleeding complications against the costs and morbidity of diagnostic tests and invasive interventions. This Practice Bulletin is a targeted revision to reflect limited changes to information about new estimates for thrombocytopenia in pregnancy and the risk of recurrence of fetal-neonatal alloimmune thrombocytopenia in subsequent pregnancies, and to provide new information on the level of thrombocytopenia that permits regional anesthesia.