ObjectiveThis study was designed to estimate the predictive value of the first neonatal platelet count for the second neonate in women with immune thrombocytopenic purpura (ITP).MethodsData of 34 patients, repeatedly pregnant while they had ITP, were prospectively collected in two study centers between 1984 and 1995. The main outcome measure was neonatal thrombocytopenia.ResultsEarly neonatal platelet counts (ie, umbilical cord count or count during the first 24 hours of life) between siblings were correlated (r= .73; 95% confidence interval (CI) for the correlation coefficient 0.52, 0.86). Severe thrombocytopenia (less than 50 × 109/L) at birth did not occur in any of the 27 siblings of infants with birth platelet levels above 50 × 109/L. Also the second sibling's nadir neonatal platelet counts during the first 2 weeks of life were correlated with those of the first sibling (r= .76; 95% CI for the correlation coefficient 0.58, 0.88). In those cases in which the first sibling had a lowest platelet count above 100 × 109/L (n= 19), the second sibling never became thrombocytopenic.ConclusionThe platelet count of the first sibling can be used to counsel women with ITP, and may be helpful in their management.