Type of test Blood

Normal findings

< 1% of red blood cells (RBCs)

Test explanation and related physiology

 Fetal hemoglobin may be present in the mother’s blood because of fetal–maternal hemorrhage (FMH), which causes leakage of fetal cells into the maternal circulation. When large volumes of fetal blood are lost in this way, neonatal outcomes can be serious and potentially fatal. Massive FMH may be the cause of about 1 in every 50 stillbirths.

Leakage of fetal RBCs can begin any time after the middle of the first trimester. It presumably results from a breach in the integrity of the placental circulation. As pregnancy continues, more women will show evidence of fetal RBCs in their circula tion; by term, about 50% will have detectable fetal cells. Most of these, however, are the result of very small leaks. In 96% to 98% of pregnancies, the total fetal blood volume lost in this way is 2 mL or less. Small leaks are not implicated in intrauterine death.

Risk factors correlated with the increasing risk of massive FMH include maternal trauma, placental abruption, placental tumors, third-trimester amniocentesis, fetal hydrops, pale fetal organs, antecedent sinusoidal fetal heart tracing, and twinning. Having one or more of these features should be an indication for fetal hemoglobin testing.

The standard method of detecting FMH is the Kleihauer Betke test. The flow cytometric method for fetal hemoglobin determination offers several advantages over the traditional Kleihauer–Betke method. This more objective method has been shown to improve sensitivity, precision, and linearity over traditional methods.

FMH becomes of even greater significance when the mother is Rh negative because this is the mechanism through which Rh sensitization could develop if the fetus has paternal Rh-positive blood cells.

This test is often performed on women who have delivered a stillborn baby to see if FMH was a potential cause of fetal death.

Interfering factors

 • Any maternal condition (e.g., sickle cell disease) that involves persistence of fetal hemoglobin in the mother will cause a false positive.

• If the blood is drawn after C-section, a false positive could occur. Vaginal delivery results in higher frequency of detection of FMH.

 Procedure and patient care

• See inside front cover for Routine Blood Testing.

• Fasting: no

• Blood tube commonly used: red

• Provide emotional support in the event this test is performed after a stillborn delivery.

Abnormal findings

- Fetal–maternal hemorrhage

- Hereditary persistence of fetal hemoglobin

- Intrachorionic thrombi