Vol. 19: Spring 2001
Prenatal Diagnosis
Invasive Prenatal Diagnostic Techniques -
Percutaneous umbilical blood sampling (PUBS)
The first report of an ultrasound guided percutaneous technique (i.e. through the mother's skin by needle puncture) to enter the umbilical cord appeared in 1983. The ability to obtain fetal blood samples opened up many new diagnostic possibilities. For example, the fetal blood can be used to diagnose hemoglobinopathies and fetal infection, for obtaining rapid chromosome analysis, to evaluate fetal hydrops and fetal acid-base status in growth restriction and diagnose erythroblastosis fetalis. Actual treatment of fetal anemias can be accomplished by direct intravascular transfusion by the percutaneous route.
PUBS involves obtaining a sample of fetal blood by placing a needle into the umbilical vein, most often where it inserts into the placenta and is least mobile. While fetal blood sampling has been obtained from the umbilical artery, the umbilical vein is preferred because it is larger and is less likely to cause a fetal bradycardia when punctured. Once the needle has been guided into the umbilical vein, fetal blood is aspirated into a syringe. In order to confirm that the sample is fetal in origin, the mean corpuscular volume (MCV) of the sample should be assessed. The MCV of a sample of fetal blood should be above 100 fL.
Complications/risks of percutaneous umbilical blood sampling (PUBS)
The most critical factor related to the safety of the procedure is operator experience. Reported fetal loss rates for PUBS are 7.2% (96/1328) overall, and 3% in a low risk group (20/660). In addition to fetal loss, other complications associated with PUBS include bleeding from the puncture site in the umbilical cord, cord hematomas, transient fetal bradycardia, infection, and feto-maternal hemorrhage. Bleeding from the cord puncture site is the most common complication and is usually self-limited. Fetal bradycardia, which lasted for a short time in the majority of cases, occurred after 9% of the procedures. In a report on diagnostic cordocenteses and intravascular transfusions, bleeding was observed from the umbilical cord puncture site in 29% of cases. Although the duration of bleeding was significantly longer after arterial puncture than after puncture of the umbilical vein, the blood loss was not clinically significant in any of the cases. The incidence of a clinically significant fetal bradycardia was 6.6%.
Clinical uses of PUBS
Based on the available data, it is clear that PUBS is a riskier procedure than CVS or amniocentesis. Currently, PUBS is offered only where there is no alternative to achieve a timely diagnosis. Fortunately, there have been major inroads in cytogenetic, molecular, and ultrasound techniques that have diminished the need for fetal blood-based diagnosis. For example, the use of PUBS for rapid fetal chromosome analysis for trisomies 13, 18, and 21 has been largely replaced by fluorescence in-situ hybridization (FISH) techniques applied to amniocytes, which can provide chromosome results within 48 hours. Placental biopsy carries a very low risk and villi obtained throughout gestation can be processed within 48 hours. Performing PCR-based assays on amniotic fluid specimens can make a number of fetal viral diagnoses. Lastly, Doppler studies have supplanted the need to sample the fetal circulation for assessment of fetal acid-base status.
Nonetheless, it is clear that PUBS has played a major role in the history of prenatal diagnosis and it is still the method of choice for lifesaving diagnostic and therapeutic measures in erythroblastosis fetalis. Other potential uses for PUBS include assessment of the fetal platelet count in cases of alloimmune thrombocytopenia and in idiopathic thrombocytopenia purpura.
The Genetic Drift Newsletter is not copyrighted. Readers are free to duplicate all or parts of its contents. The Genetic Drift Newsletter is published semiannually by the Mountain States Regional Genetic Services Network for associates & those interested in Human Genetics. In accordance with accepted publication standards, we request acknowledgement in print of any article reproduced in another publication. The views expressed in the newsletter do not necessarily reflect local, state, or federal policy. For additional information, contact Carol Clericuzio, M.D., Editor, Department of Pediatrics, The University of New Mexico, Albuquerque, NM, 87131
Prenatal Diagnosis - Table of Contents
Introduction
Prenatal Diagnosis of Single Gene Disorders
Down Syndrome Maternal Serum Screening
Fetal Ultrasound
Amniocentesis
Chorionic Villus Sampling
Percutaneous Umbilical Blood Sampling
Preimplantation Genetic Diagnosis
The Future of Prenatal Diagnosis
TABLE: Timetable for Prenatal Diagnostic/Screening Procedures
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