Fetomaternal hemorrhage: incidence, risk factors, time of occurrence, and clinical effects.

Most women have only very small amounts of fetal blood in their circulations following pregnancy and delivery: the volume is less than 0.5 mL of whole blood in 93 percent of women, less than 1 mL in 96 percent, and less than 2 mL in 98 percent. FMH of 30 mL or more occurs in just 3 of 1000 women. When the FMH was 150 mL or more, 15 of 41 infants did not survive Rh-negative women with FMH of more than 30 mL of Rh-positive whole blood are at increased risk of Rh immunization, and thus the outcome of their future pregnancies also may be affected. ABO-compatible fetal red cells that have entered the maternal circulation have a life span similar to that of adult cells. ABO-incompatible fetal red cells may be cleared rapidly, but in some cases they circulate for weeks. Most FMHs of 30 mL or more occur before labor, delivery, or cesarean section. The majority occur with minimal clinical signs and symptoms in apparently normal pregnancies. The identification of postpartum Rh-negative women who have 30 mL or more of Rh-positive fetal blood in their circulation is important so that sufficient RhIG for immune suppression can be administered. It appears that more than one-half of women with FMH of 30 mL or more would not be identified if protocols were adopted to test only women in pregnancies considered to be at high risk.