Rh hemolytic disease of the newborn: using incidence observations to evaluate the use of RH immune globulin.

During the 1970s, the crude incidence of Rh hemolytic disease of the newborn (RhHDN) in the United States dropped 65 per cent---from 40.5/10,000 total births in 1970 to 14.3/10,000 total births in 1979. Since the risk of RhHDN is directly associated with birth order, changes both in birth order-specific RhHDN incidence rates and in the birth order distribution would be expected to influence crude RhHDN incidence. Review of birth order-specific incidences ascertained in Connecticut, 1972-1977, shows that incidence was stable for first and second births, whereas it dropped significantly for third and fourth births. Due to the nearly complete use of Rh immune globulin (RhIG) in Connecticut during these years, the drop in these specific rates is best attributed to this prevention strategy. Nationwide, during the 1970s, the annual percentages of fourth and higher order births decreased from 18 per cent to 10 per cent. We estimate that some 30 per cent to 40 per cent of the 1970-1979 drop in the crude rate is attributed to changes in the birth order distribution, whereas 60 per cent to 70 per cent of the drop is attributed to RhIG. However, comparison of pooled incidences for 1972-1974 and 1975-1977 observed in Connecticut for first births and births to 15-19 year olds showed significant increases for both groups in 1975-1977. Further study of specific trends from other areas is needed to confirm these observations. Also needed is investigation of likely causes of sensitization among these teenaged, primiparous women.