Abstract Background Anti-D immunoglobulin administered in the early third trimester to Rhesus D negative women with Rhesus D incompatibility is widely assumed to provide protection against silent fetomaternal haemorrhage until delivery. However, previous studies in singleton pregnancies demonstrated that passively acquired anti-D is frequently no longer detectable by the time of delivery. The aim of this study was to assess whether the proportion of women with detectable passively administered a