Clinical significance of anti-M in pregnancy and self-reported ancestry: The BEST collaborative study

Anti-M antibodies were described as rarely eliciting hemolytic disease of the fetus and newborn (HDFN) in pregnancy. More recent literature, primarily from Asia, reports severe HDFN attributed to maternal anti-M. The objective of this study is to determine if there is a difference in the frequency and/or severity of anti-M HDFN among subjects who self-report as Asian compared to non-Asian.

Retrospective data were collected from 18 sites. Fetuses/neonates affected by HDFN were categorized as unaffected, mild, moderate, or severe based on clinical management and outcomes.

Anti-M was identified in 557 obstetric patients. Among 332 neonates with complete data for analysis, anti-M was detected in isolation during the pregnancies of 210 (63.3%) neonates; 198 (94.3%) were unaffected by HDFN. The overall frequency of HDFN was 6.5% (2/31) in Asian patients and 5.6% (10/179) in non-Asian patients. The frequency of severe HDFN was 6.5% (2/31) among Asian patients and 0.6% (1/179) among non-Asian patients. Among non-Asian patients, eight (4.5%) were mild and one (0.6%) was moderate. Of 63 pregnancies with anti-M and ABO incompatibility, 6 (9.5%) were associated with mild HDFN and 57 (90.5%) were unaffected. Of 49 pregnancies with anti-M and non-ABO antibody(s), 6 (12.2%) were associated with mild HDFN and 43 (87.8%) were unaffected. One case (10.0%) of HDFN (mild) occurred in the presence of anti-M, additional antibody(s), and ABO incompatibility.

In this study, severe anti-M HDFN was rare in non-Asian subjects. There is a signal that Asian patients are at greater risk for severe anti-M HDFN.