Design: Population modelling from laboratory antibody screens with Monte Carlo simulation
Population: 8,809,993 US pregnancies screened 2010–2021
Primary outcome: Estimated national rate of severe haemolytic disease of the fetus and newborn
Effect: 2.4 per 100,000 pregnancies (95% CI 2.0–2.9); 3.8% of critical-titre pregnancies

Researchers used prenatal red cell antibody screens from a national US laboratory, covering about 8.8 million pregnancies from 2010 to 2021, reweighted them to the US birth population and ran simulations to estimate how often clinically significant alloimmunisation ends in severe haemolytic disease of the fetus and newborn.

Any high-risk antibody (anti-D, anti-K or anti-c) was found in about 687 per 100,000 pregnancies, but only about 64 per 100,000 reached a critical titre. The estimated rate of severe disease was 2.4 per 100,000 pregnancies, and about 4% of pregnancies with a critical titre were expected to end in severe disease.

The notable detail is the antibody mix. Anti-D remains the most common antibody found, but anti-K reached critical titres twice as often as anti-D in absolute terms. Anti-K is not prevented by anti-D prophylaxis, and Kell disease can suppress fetal red cell production, so titres can underestimate risk.

These are modelled US estimates and Indian rates will differ, particularly where anti-D prophylaxis coverage is incomplete. The practical message holds anywhere: a positive screen needs the antibody named and followed, not just noted.

Identify the specific antibody whenever an antenatal screen is positive.
Refer pregnancies with anti-K early to fetal medicine; titres can understate the risk with Kell.
Check paternal antigen status where it would change surveillance.
Keep offering anti-D prophylaxis to RhD-negative women; it does not protect against anti-K or anti-c.
Most women with an antibody will not have an affected baby; counsel with that in mind.

Why it matters

With anti-D prophylaxis working, the antibodies left causing severe disease are the ones prophylaxis cannot touch.

The statistics, in plain English

The 2.4 per 100,000 figure is a model output built on several assumptions, including paternal antigen frequencies and continuation rates, not a count of affected babies. Its confidence interval (2.0 to 2.9) reflects simulation uncertainty, not the uncertainty in those assumptions.