The Principles of Clinical Cytogenetics - Extra Materials - Springer

Prenatal Cytogenetics 287
fetuses. MCC occurs more often in cultured cells than in direct preparations, thus underscoring the
importance of using both methods in a full CVS cytogenetic analysis. In one report (115), the rate of
MCC was significantly higher in specimens obtained by the transcervical method (2.16%) than in
samples obtained by the transabdominal method (0.79%).
A note of caution is prudent here. Generally, when there is a discrepancy between the direct and
the cultured preparations, a subsequent amniocentesis is considered to provide the “true” result. However,
a case of mosaic trisomy 8 reported by Klein et al. (127) illustrates the fact that a true low-level
tissue-specific mosaicism can exist. In this case, the CVS showed a normal direct preparation and
mosaic trisomy 8 in culture. Subsequent amniocentesis showed normal chromosomes, but peripheral
blood cultures of the newborn showed trisomy 8 mosaicism. Therefore, when considering amniocentesis
or percutaneous umbilical blood sampling (PUBS) as follow-up studies because of possible
CPM observed in CVS, one needs to weigh factors such as the specific aneuploidy involved, the
likelihood of detecting it using a given sampling technique, and the risks of the additional invasive
procedure.
Specimen Requirements
The minimum amount of chorionic villus material necessary to obtain diagnostic results and the
transport medium should be established in advance with the laboratory. In general, a minimum of
10 mg of tissue is needed to obtain both a direct and a cultured cell result; 20 mg is ideal. If possible,
the specimen should be viewed through a dissecting microscope to ensure that villi are present. The
specimen should be transported at ambient temperature to the cytogenetics laboratory as soon as
possible.
Percutaneous Umbilical Blood Sampling
Risks, Limitations, and Benefits
Percutaneous umbilical blood sampling is also known as periumbilical blood sampling, fetal blood
sampling, or cordocentesis. The largest series in the literature regarding risks of PUBS (128) included
outcomes of 1260 diagnostic cordocenteses among 3 fetal diagnosis centers and 25 practitioners. A
fixed-needle guide was used in this study, and prospective data were compared to the published
experience of large centers that use a freehand technique, where a 1–7% fetal loss rate has been
reported. The procedure-related loss rate at a mean gestation of 29.1 ±5 weeks at the time of sampling
was 0.9%, leading to the conclusion that technique is a variable in the loss rate for cordocentesis.
PUBS experience at an earlier gestation was described by Orlandi et al. (129) in 1990, who pointed
out that although cordocentesis was a technique largely confined to the middle of the second trimester
to term, in their experience it could be performed as early as the 12th week with acceptable results.
They evaluated the outcomes of 500 procedures performed between 12 and 21 weeks for thalassemia
study (386), chromosome analysis (97), fetomaternal alloimmunization (10), and infectious disease
diagnosis (7). One practitioner performed the procedures, and the volume of blood obtained ranged
from 0.2–2.0 mL, depending on the gestational age. Of the 370 pregnancies not electively terminated
and for which outcome information was available, the fetal loss rate was 5.2% for fetuses of 12–18
weeks’ gestation and 2.5% between 19 and 21 weeks. Indicators of adverse outcome included cord
bleeding, fetal bradycardia, prolonged procedure time, and anterior insertion of the placenta. Fetal
bradycardia is a commonly reported complication after PUBS and is associated with a higher
likelihood of fetal loss. In a review of 1400 pregnancy outcomes after PUBS, the overall incidence
of recognizable fetal bradycardia was estimated at 5% (130). It was significantly more likely to occur
when the umbilical artery was punctured. Boulot et al. (131) performed 322 PUBSs and noted fetal
bradycardia, usually transitory, in 7.5% of their cases. Fetal bradycardia occurred in 2.5% of cases
with normal outcome and in 12.5% of cases of fetal loss in one study (129), and in another, 11 of 12
fetal losses were associated with prolonged fetal bradycardia (130).