Relevance of new recommendations on routine antenatal prevention of rhesus immunization: an appraisal based on a retrospective analysis of all cases observed in two French administrative areas of 3 million inhabitants.

OBJECTIVE: To assess the potential impact of new guidelines recommending routine antenatal prophylaxis at 28 weeks of pregnancy on incidence, consequences and cost of rhesus immunization.

STUDY DESIGN: All rhesus immunizations of 224,500 ongoing pregnancies in two neighbouring administrative areas in France between 2000 and 2006 were enrolled in this retrospective study. To determine the aetiology of immunization and to specify when sensitization occurred, we searched sensitizing events between the last negative and the first positive red-cell antibody test results. Perinatal consequences and costing were also analyzed.

RESULTS: From 138 rhesus negative women bearing anti-D antibodies, none had received routine prophylaxis at 28 weeks. 37% were primary immunizations and 63% were reactivating former immunization. 63% sensitizations occurred after unprovoked foetal-maternal haemorrhage, mostly after 28 weeks (54%). Twenty-five (18.1%) sensitizations resulted from inappropriate management of existing prophylaxis. Immigrants with previously acquired antibodies accounted for 10% of cases. There was no foetal demise and none born before 28 weeks among our 140 babies. Only 25% required intensive care, mostly those born to mothers reactivating immunization, with an overall good perinatal outcome. Systematic 28-week prophylaxis would have cost about euro 2.5 million to reduce overall cost of immunizations by euro 0.6 million.

CONCLUSIONS: The incidence of rhesus immunization in our population was low at 0.41 per thousand. Routine antenatal prophylaxis could have avoided 54% of these immunizations but expected perinatal benefits are low, as newborns with the worst issue were born to mothers with unavoidable immunizations. Therefore the cost-effectiveness of this strategy is doubtful.