Anti-D concentrations in fetal and maternal serum and amniotic fluid in rhesus allo-immunised pregnancies.

Objective To investigate the relation between anti-D concentrations in maternal serum, fetal serum and amniotic fluid, and the development of fetal anaemia. Design Observational cross sectional and longitudinal study. Setting Regional referral centre. Subjects Sixty-one women undergoing fetal blood sampling at 19 to 36 weeks' gestation for fetal blood and haematocrit estimation for the management of Rh (D) allo-immunisation. Thirty-eight pregnancies (7 with an Rh (D) negative fetus) were tested only once but the rest had two to five fetal blood samplings. Interventions Ultrasound guided fetal blood sampling and amniocentesis, and automated analysis of anti-D antibody quantitation. Results There were strong correlations between maternal serum, fetal serum and amniotic fluid anti-D concentrations. Analyses of both longitudinal and cross sectional data demonstrated a decrease of the maternal/fetal serum anti-D ratio with gestation. In pregnancies with Rh (D) negative fetuses the maternal/fetal anti-D ratio was significantly lower (P<0.0001) than in those with Rh (D) positive fetuses. The degree of fetal anaemia (delta haematocrit) was correlated with maternal serum and amniotic fluid anti-D concentrations (r = -0-55, n = 54, P<0.0001; r = -0.57, n = 44, P<0.0001, respectively) but there was a weaker correlation with fetal serum anti-D (r = 0.37, n = 54, P<0.01). Conclusion Anti-D concentrations in maternal serum, fetal serum and amniotic fluid are correlated with fetal anaemia. The decrease in maternal/fetal anti-D ratio with gestation suggests an increase in placental permeability for anti-D with advancing pregnancy.