NAITP is suspected if a neonate unexpectedly presents with petecchiae and is severely trombocytopenic. The thrombocytopenia is caused by maternal alloantibodies to platelet-specific antigens, most often anti-HPA 1a. A mother who has previously given birth to an affected child is usually considered at high risk for giving birth to a trombocytopenic child also in the subsequent pregnancy. The aim of the present study was to compare to what extent quantification of anti-HPA 1a versus obstetric history can predict the severity of thrombocytopenia in new-borns. Anti-HPA 1a was repeatedly quantified in 152 immunized pregnant women who were included in a large Norwegian prospective study. Sixty-eight of the women gave birth to thrombocytopenic children of which 42 suffered severe thrombocytopenia (plt. count <50x109/l). Ten women had previously given birth to children with symptomatic NAITP. Five of these did not give birth to children with severe thrombocytopenia in their subsequent pregnancy. Positive predictive values for thrombocytopenia (<150x109/l) were 0.75 using obstetric history and 0.80 for anti-HPA 1a antibody level above 100 Arbitrary Units (AU). Positive predictive values for severe NAITP (<50x109/l) were hence 0.50 and 0.51 for obstetric history and antibody level. However, the diagnostic sensitivity for obstetric history was only 0.12, whereas anti-HPA 1a level above 100 AU had a diagnostic sensitivity of 0.88. In addition, we found that the level of anti-HPA1a tend to increase in first pregnancy and decrease in subsequent non-compatible pregnancies. This observation is in accordance with the fact that half of the women, who gave birth to NAITP children in their first pregnancy, did not get an affected child in their next pregnancy. In conclusion, obstetric history only identifies a few of the affected cases since all first time cases of sever NAITP are missed. Both obstetric history and antibody level has comparable positive predictive values for severe NAITP. However, the level of anti-HPA 1a antibody would be the best marker to identify severe NAITP cases because of independence of parity and obstetric history.