Please cite this paper as: Madani K, Kamphuis M, Lopriore E, Porcelijn L, Oepkes D. Delayed diagnosis of fetal and neonatal alloimmune thrombocytopenia: a cause of perinatal mortality and morbidity. BJOG 2012;119:1612-1616. Objective To evaluate the rate and consequences of a late or missed diagnosis of fetal and neonatal alloimmune thrombocytopenia (FNAIT). Design Retrospective analysis of prospectively collected data of a national cohort. Setting National referral centre for fetal therapy in the Netherlands. Population Twenty-six women with pregnancies complicated by FNAIT and at least one previous pregnancy with a thrombocytopenic child. Methods Retrospective analysis of data from our electronic FNAIT database. In a consecutive cohort managed between July 2008 and July 2010, timing of first diagnosis of FNAIT was correlated to severity and outcome in the subsequent pregnancies. Main outcome measures Occurrence of delayed diagnosis of FNAIT, and possibly associated intracranial haemorrhage (ICH). Results In four of 26 pregnancies, timely diagnostic testing for FNAIT was not performed despite fetal or neonatal thrombocytopenia or ICH. Down syndrome, dysmaturity and birth trauma were perceived to be the cause of the thrombocytopenia/ICH. In two of these four subsequent, untreated pregnancies, severe fetal ICH occurred. The other 22 women were treated for FNAIT using intravenous immunoglobulin, all children are alive and well. Conclusions All neonates with thrombocytopenia at birth should be evaluated for FNAIT. Missing this diagnosis can have severe consequences for subsequent pregnancies. [ABSTRACT FROM AUTHOR]