Fetomaternal hemorrhage (FMH) is the transfer of fetal red blood cells into the mother’s bloodstream. This mixing occurs to some degree in nearly all pregnancies, often involving volumes so small they are clinically insignificant. While minimal transfers are common and generally harmless, a large volume of fetal blood entering the maternal system is a rare but medically significant event. The volume of blood loss dictates the severity, which can range from a minor event to one causing severe fetal anemia, distress, or even death. Quantifying this blood transfer is necessary to determine the correct medical intervention for both the mother and the developing fetus.
How Fetal Blood Enters Maternal Circulation
The placenta normally separates the maternal and fetal circulatory systems, allowing for the exchange of nutrients, oxygen, and waste without the blood cells mixing. This separation is maintained by placental tissue layers lining the chorionic villi, where fetal blood circulates in capillaries close to the maternal blood flow.
FMH occurs when this delicate placental barrier is physically disrupted. This breach allows red blood cells to pass from the higher-pressure fetal side into the lower-pressure maternal side. A significant consequence involves the Rhesus (Rh) blood group system: if an Rh-negative mother is exposed to Rh-positive fetal red blood cells, her immune system may recognize the Rh protein as foreign and begin producing antibodies, a process known as sensitization.
Situations That Increase Risk
FMH can result from trauma, obstetric procedures, or spontaneous events. Blunt external abdominal trauma, such as from a motor vehicle accident or a fall, is a frequent cause of large bleeds because the force can physically tear placental connections. The risk of FMH increases with the severity of the trauma, necessitating immediate evaluation even without visible external injury.
Obstetric procedures involving the uterine or placental environment also carry a risk of breaching the barrier. Invasive diagnostic tests like amniocentesis and cordocentesis can directly cause a small amount of fetal blood transfer. Procedures intended to manually reposition the fetus, such as external cephalic version (ECV), involve pressure on the uterus that may lead to placental disruption.
Spontaneous events during pregnancy can also trigger significant FMH without external force. Conditions involving abnormal placental separation, such as placental abruption or placenta previa, can result in a sudden and massive bleed. However, in many cases of significant FMH, no clear precipitating factor is identified, making the condition unpredictable.
Measuring the Volume of Blood Loss
Quantifying the volume of fetal blood transferred is necessary for managing FMH, especially for Rh-negative mothers, as the volume determines the required dose of prophylactic treatment. The Kleihauer-Betke (KB) acid elution test has traditionally been used to estimate this volume by distinguishing fetal red blood cells (RBCs) from maternal RBCs.
The principle of the KB test relies on the difference in hemoglobin composition: fetal hemoglobin (HbF) resists acid elution, while adult hemoglobin is washed away. This process leaves fetal cells stained pink and maternal cells as pale “ghosts” under a microscope, allowing a technician to count the percentage of fetal cells in the maternal blood sample. However, the KB test is labor-intensive and its accuracy can be limited by human error and the presence of adult cells containing persistent HbF.
Flow cytometry is a more modern and precise method for quantifying FMH. It uses fluorescently labeled antibodies specific to fetal red cell proteins or hemoglobin to rapidly analyze thousands of cells. This offers a more objective and accurate measurement of the fetal cell percentage. This precision is important for calculating the precise, higher dose of anti-D immunoglobulin required in cases of massive hemorrhage. The measured percentage is then used with the mother’s blood volume to estimate the total bleed volume in milliliters.
Managing and Treating the Condition
Management of FMH has two primary goals: preventing maternal immune sensitization in Rh-negative mothers and addressing fetal blood loss. For an Rh-negative mother, treatment involves administering Rh(D) Immune Globulin (RhoGAM or RhIg). This product contains antibodies that neutralize Rh-positive fetal red blood cells in the mother’s circulation before her immune system can mount a long-term response.
The standard dose of RhoGAM (300 micrograms) protects against a bleed of up to 15 milliliters of fetal red blood cells. If quantification tests reveal a larger bleed volume, the dose must be increased proportionately. Accurate measurement ensures the mother receives adequate RhIg, preventing the formation of permanent antibodies that could attack an Rh-positive fetus in a subsequent pregnancy.
Fetal management assesses the degree of anemia and hypoxia caused by the blood loss. Physicians monitor the fetus using ultrasound and Doppler studies, specifically measuring the peak systolic velocity in the middle cerebral artery (MCA-PSV), a reliable indicator of fetal anemia. If severe anemia occurs in a premature fetus, an intrauterine blood transfusion may be performed by injecting blood directly into the umbilical cord vein. If the fetus is near term and experiencing severe distress or hydrops, an emergency delivery allows for immediate postnatal resuscitation and transfusion.