Rhesus negative pregnancy

A basic understanding of the pathophysiology is essential to understand the management of Rhesus negative pregnancies

• along with the ABO system the Rhesus system is the most important antigen found on red blood cells. The D antigen is the most important antigen of the rhesus system
• around 15% of mothers are rhesus negative (Rh -ve)
• if a Rh -ve mother delivers a Rh +ve child a leak of fetal red blood cells may occur
• this causes anti-D IgG antibodies to form in mother
• in later pregnancies these can cross placenta and cause haemolysis in fetus
• this can also occur in the first pregnancy due to leaks

Prevention

• test for D antibodies in all Rh -ve mothers at booking
• NICE (2008) advise giving anti-D to non-sensitised Rh -ve mothers at 28 and 34 weeks
• the evidence base suggests that there is little difference in the efficacy of single-dose (at 28 weeks) and double-dose regimes (at 28 & 34 weeks). For this reason the RCOG in 2011 advised that either regime could be used 'depending on local factors'
• anti-D is prophylaxis - once sensitization has occurred it is irreversible
• if event is in 2nd/3rd trimester give large dose of anti-D and perform Kleihauer test - determines proportion of fetal RBCs present

Anti-D immunoglobulin should be given as soon as possible (but always within 72 hours) in the following situations:

• delivery of a Rh +ve infant, whether live or stillborn
• any termination of pregnancy
• miscarriage if gestation is > 12 weeks
• ectopic pregnancy (if managed surgically, if managed medically with methotrexate anti-D is not required)