Background
Despite that anti-D prophylaxis regimens to prevent alloimmunisation during pregnancy are used for fifty years, and are shown effective, anti-D is the most common red cell antibody causing severe hemolytic disease of the fetus and newborn (HDFN).1 Intrauterine transfusions are in the majority of cases due to anti-D immunisation.2
Postnatal anti-D immunoglobulin prophylaxis to RhD negative women with a newborn blood typed RhD positive was introduced in the late 1960s and reduced the risk of immunisation of RhD negative women from approximately 15% to 1-1.5%. 3,4In the 1990s many countries added routine antenatal anti-D prophylaxis (RAADP) in the third trimester to all RhD negative women, further minimising the risk of immunisation to an incidence of 0.2-0.4%.5 During the last decade fetal RHD -typing combined with targeted RAADP only to those carrying an RHD positive fetus, has been proven effective and is implemented in many countries.6 The strategies of doses and timing of administration of RAADP vary between countries, one single dose of anti-D; 1000-1500 IU in gestational week 28-30 or two doses of 500-625 IU at 28 and 34 weeks are the most common routines.7,8,9 A second or third dose anti-D is given postdelivery, usually a dose of 1000-1500 IU. In some programs, testing of fetomaternal hemorrhage (FMH) is recommended at delivery, to determine if additional doses of anti-D are required.10
It is known, that one single dose anti-D in gestational week 28 is not detectable at delivery in a proportion of women, varying from 39-56% and up to 78% for those who deliver after 40 weeks of gestation.7,8 thus not protecting them from immunisation at the end of the third trimester, when the risk of immunisation is increased.9,11,12 The two-dose strategy gives a higher rate of detectable anti-D at term, 85%, but is associated to higher costs and lower compliance of receiving the two doses.13,14
In Sweden the recommendations are to perform fetal RHD screening in the first trimester, and to administer targeted RAADP, 1500 IU in gestational week 28 and a second dose of anti-D 1500 IU postdelivery,15,16 and at situations carrying an increased risk of fetomaternal hemorrhage during pregnancy.
Today, when fetal RHD typing is widely implemented and theRHD type of the fetus is known with high accuracy6 an alternative strategy could be to administer two RAADP doses, at gestational week 28 and 38, the latter to cover term and post term of the pregnancy. If the postdelivery anti-D dose could be excluded, this strategy could be cost effective and have the potential of high compliance.
The aim of this study was to retrospectively analyse the proportion of women with undetectable levels of prophylactic anti-D at the time of delivery after one dose of RAADP (1500 IU) at gestational week 28. Secondly, in a prospective study, to investigate if a strategy with administration of the second dose of anti-D in gestational week 38, instead of postdelivery, would improve the protection at term and if the protection would be enough after delivery.