Diagnostic ultrasound ( PDFDrive )

1108 PART IV Obstetric and Fetal Sonography
A B C
FIG. 31.15 Diagnostic Procedures. (A) Transabdominal chorionic villus sampling. Transverse image of needle entering the placenta. (B)
Transcervical chorionic villus sampling. Sagittal image shows a catheter passing through the cervix and into the placenta. Note the full bladder and
amniotic cavity. (C) Amniocentesis. Transverse ultrasound image shows needle entering the amniotic luid. See also Video 31.5.
and the stylet removed (Fig. 31.15A). A 20-mL syringe with
about 2 mL of cell-transport medium is connected to the needle.
he needle is then guided back and forth through the placenta
while suction is intermittently applied to the syringe. he needle
is withdrawn and the cell-transport medium is drawn though
the needle to clear any remaining villi. he specimen is examined
under a dissecting microscope for adequacy. For transcervical
CVS (Fig. 31.15B), a 5.7-Fr lexible CVS cannula with a rigid
metal introducer is guided into the cervix under direction
visualization, and then into the placenta under sonographic
guidance (Fig. 31.15B). he introducer is withdrawn and a 20-mL
syringe with about 2 mL of transport medium is connected to
the cannula. Suction is intermittently applied to the cannula as
it is slowly withdrawn from the placenta. As with all fetal procedures,
the fetal heart rate is documented ater the procedure.
Women who are Rh negative and who have an Rh-positive partner
are given RhoGAM.
he CVS specimen can then be tested with luorescent in situ
hybridization (FISH), direct cytogenetic analysis, or cultured
villi for numerous conditions. 240,241 One concern with CVS is
that there is a discrepancy between the prenatal cytogenetic
diagnosis from placental source and the fetal karyotype. he
U.S. Collaborative Study on CVS and other large registries have
found that a successful genetic diagnosis can be obtained in
99.7% of cases, with an FPR of 11 per 10,000 pregnancies. 242,243
here were no diagnostic errors involving trisomy 13, 18, or 21
in these studies. 242 Maternal cell contamination occurs in 0.8%
to 2.2% of cases. 242,244 Approximately 0.5% of CVS samples will
have an ambiguous inding that needs conirmation by amniocentesis.
245 Conined placental mosaicism is seen in 0.7% to
1.6% of cases 242-244 and may be caused by meiotic errors with
subsequent “trisomy rescue” or by mitotic errors in the developing
morula. 246 Conined placental mosaicism can also cause a falsepositive
cell-free DNA result and, in some cases, diagnostic testing
by amniocentesis is suggested. 247
A recent meta-analysis reports a weighted pooled procedure–
related miscarriage at less than 24 weeks for CVS as 0.22% (95%
CI, −0.71%-1.16%). 248 he Canadian Collaborative CVS-
Amniocentesis Clinical Trial group recruited women at less than
12 weeks’ gestational age and randomized them to CVS in the
irst trimester or amniocentesis at 15 to 17 weeks’ gestational
age. No signiicant diferences were seen in fetal loss rates. 249
Caughey et al. 250 compared CVS, amniocentesis, and control
groups of women at similar gestational ages who declined
intervention and found no signiicant diferences in the adjusted
loss rates between the CVS and amniocentesis groups in the
most recent study period reported. Early concerns about an
increased risk of oromandibular-limb hypogenesis syndrome 251
have been not been upheld. 252 A report by the World Health
Organization on the safety of CVS found that the risk of limb
reduction defects by week was comparable to that of the general
population, except at 8 weeks’ gestational age, when it was elevated
with CVS. 253
Genetic amniocentesis is typically performed between 15
and 20 weeks’ gestation (Fig. 31.15C). Sonographic guidance is
used for location of a luid pocket. Most oten a 22-gauge 3.5-inch
long spinal needle is used (Fig. 31.15C, Video 31.5). For larger
patients a longer needle may be required. he placenta is avoided
if possible, but performing the procedure through the placenta
is thought to be safe. Typically 15 to 20 mL of luid is removed
(with a rule of thumb being 1 mL per week of gestation). When
amniocentesis is performed in multiple gestations, care is taken
to approach each sac separately. he reported procedure-related
loss rate before 24 weeks’ gestation in a recent meta-analysis was
0.11% (95% CI, −0.04%-0.26%). 248 he heterogeneity of studies
on diagnostic testing limits a precise estimation of procedurerelated
loss for CVS or amniocentesis; however, the added
procedure-related risk following is quite low (0.2% and 0.1%). 254
he commonly quoted procedure loss rate is “less than 1 in 300
to 500 procedures.” 254 Leakage of amniotic luid may occur ater
a genetic amniocentesis, although favorable pregnancy outcomes
are seen in more than 90% of women with expectant management.
255,256 Amniocentesis between 11 and 13 weeks’ gestation
has been associated with increased pregnancy loss rates,