Design: Prospective, non-interventional natural history study
Population: 14,114 pregnant women without prior FNAIT at 28 sites
Primary outcome: Anti-HPA-1a alloimmunisation in higher-risk women by 10 weeks post partum
Effect: 1.7% HPA-1b/1b; 2 of 19 higher-risk women alloimmunised (11%, 1.3% to 33.1%)

This prospective natural history study screened 14,114 pregnant women with no history of fetal-neonatal alloimmune thrombocytopenia (FNAIT) at 28 sites in North America and Europe. At 10 to 14 weeks it applied a sequence: maternal HPA-1 genotype, HLA-DRB3*01:01 status, anti-HPA-1a antibodies and fetal HPA-1 genotype.

Overall 1.7% were HPA-1b/1b, with variation by race and geography. Only 24 women met the full higher-risk definition. Of the 19 followed up, 2 (11%, 95% CI 1.3% to 33.1%) had anti-HPA-1a antibodies by 10 weeks post partum. A further 18 HPA-1b/1b, DRB3*01:01-positive women already had antibodies at screening.

FNAIT can cause fetal intracranial haemorrhage in a first pregnancy, which is why screening keeps being debated. This study mostly shows how few women fall into the highest-risk group and how wide the uncertainty is around their conversion rate. It is a methodology and prevalence study, not evidence that population screening improves outcomes.

Suspect FNAIT in any neonate with unexplained severe thrombocytopenia or intracranial haemorrhage, including a first baby.
Refer women with a previous FNAIT-affected pregnancy to fetal medicine early; recurrence risk is high.
Expect pre-existing anti-HPA-1a antibodies in some HPA-1b/1b women at first screening, not only new conversions.
Population HPA-1a screening is not standard practice; this study does not change that.

Why it matters

It gives the first multi-ethnic estimate of how many women sit in the highest-risk HPA-1a group.

The statistics, in plain English

Two of 19 women is 11%, but with so few women the interval runs from about 1% to 33%, so the true conversion rate is very uncertain. Five higher-risk women were lost to follow-up, which could move the estimate either way.