Objective: To analyse the proportion of RhD negative women where routine antenatal anti-D prophylaxis (RAADP) in gestational week (gw) 28 is not detectable at delivery and to investigate if a strategy with RAADP administered in gw 28 and 38 result in efficient protection at term, postterm and postdelivery. Design: A retrospective database analysis and a prospective interventional study. Setting: Antenatal centers in the Stockholm region. Sample: RhD negative women carrying an RHD positive fetus, 4280 cases evaluated retrospectively, 39 cases included prospectively. Methods: In a retrospective analysis, RhD negative women with a negative antibody screen at delivery was determined. In 39 pregnancies, quantification of anti-D was analysed before a second dose anti-D was administered in gw 38, and then weekly up to 43 weeks post gestation. Main outcome measures: The proportion of women with non-detectable anti-D at term. The concentration of anti-D measured weekly at term, postterm and postdelivery. Results: In 20,5% (856/4280, retrospective analysis) and 44% (17/39, prospective study) RAADP in gw 28 was below screening detection level, 10 IU/L at term and in 18% (7/39, prospective study) below 1 IU/L in the quantification assay. Anti-D prophylaxis administered in gw 38 showed stable protective levels of anti-D up to 30 days postdelivery, with concentration at delivery 60+34 IU/L (mean+SD). Conclusion: At least one third of the RhD negative women have non-detectable levels of RAADP given at gw 28, at term and postterm. A second dose of RAADP at gw 38 improves the protection.