AIT is associated with antenatal intracranial hemorrhage (ICH) in up to 10% of all cases. We have entered 108 maternal-fetal pairs on antenatal treatment protocols to increase the fetal platelet count (plt ct) and prevent ICH but this alters the natural history of AIT. Nonetheless we have derived 3 sets of insights into fetal AIT from these patients (pts). (A) Among 108 fetuses whose pre-treatment fetal blood sampling (FBS) took place at a mean gestational age(GA) of 25 weeks, the mean fetal plt ct was 37,000/ul (37k) but the median was 23k. 22 of 45 fetuses or 49% sampled at < 24 wks GA had plt cts ≤ 20k. The plt ct at the first FBS however was unrelated to ICH in the previous untreated sibling (sib): +ICH, n=24, plt ct at FBS = 33k compared to -ICH, n=72, plt ct = 39k. (B) 39 of 96 fetuses or 41% already had a lower plt ct at 1st FBS than their previous sib's birth plt ct. (C) In 3 cases of PlA1 incompatibility, an FBS was repeated after 3-6 wks prior to initiating any treatment. The plt cts decreased in these fetuses: 149k to 21k, 80k to 22k, and 129k to 28k. In summary, AIT: (A) manifests severe fetal thrombocytopenia early in gestation irrespective of sib history of ICH; (B) often worsens from 1 sib to the next since the initial FBS plt ct is already worse than the sib birth plt ct in > 40% of cases; and (C) worsens during gestation if untreated. These facts are important in assessing the need for antenatal management of subsequent affected pregnancies in women with AIT.