Fetal monitoring is used to assess the severity of HDFN and determine whether antenatal transfusion is warranted. Monitoring can be accomplished by:
- Doppler ultrasonography
- Amniocentesis
- Cordocentesis
Doppler Sonography
Doppler sonography is a type of ultrasound that detects and measures blood flow. Doppler sonography can be done beginning at 18 weeks. It measures the peak velocity of systolic blood flow in the fetal middle cerebral artery and is used to predict the severity of fetal anemia. The hypothesis is that a faster rate of blood flow indicates a more severely anemic fetus, with severe anemia being an indicator of fetal hydrops.
Because Doppler sonography is noninvasive and a safer alternative to amniocentesis, it has largely replaced serial amniocentesis for predicting the severity of HDFN.
Amniocentesis
Amniocentesis is a procedure used to obtain amniotic fluid for diagnostic tests. As applied to diagnosing the severity of HDFN, amniocentesis can be done beginning at approximately 28 weeks and repeated to provide serial estimates of the amount of bile pigment (bilirubin) in amniotic fluid. The process measures the difference in optical density at a wavelength of 450 nm between the observed density and an extrapolated baseline (if no excess bilirubin was being produced).
Serial values are plotted on charts (e.g., Queenan chart or extended Liley chart), which categorize HDFN into 3 zones of severity.
Cordocentesis
Cordocentesis, also known as percutaneous umbilical blood sampling (PUBS), can be done after 18 weeks gestation. PUBS is a prenatal procedure in which a fetal blood sample is removed from the umbilical cord. The sample is confirmed to be of fetal origin by a rapid alkaline denaturation test. The fetal blood can then be analyzed using routine diagnostic tests (e.g., blood group, DAT, reticulocyte count, platelet count, hemoglobin/hematocrit). Cordocentesis/PUBS can also be used to deliver intravenous transfusions (IVTs).
