Feto-maternal haemorrhage is detected by

Kleihauer - Betke test : To estimate the approximate volume of fetal blood entering into the maternal circulation. Using acid elution technique to note the number of fetal red cells per 50 low power fields If there are 80 fetal erythrocytes in 50 low power fields in maternal peripheral blood films, it represents a transplacental hemorrhage to the extent of 4 ml of fetal blood. Ref:Dutta Obs 9e pg 314.