The Principles of Clinical Cytogenetics - Extra Materials - Springer

280 Linda Marie Randolph
Table 9
Volume of Amniotic Fluid (mL) Calculated
Using All of the Values for a Given Week from Published Data
Week n Mean SD Range
10 7 29.7 11.2 18–33
11 9 53.5 16.4 64–76
12 13 58.0 23.4 35–86
13 13 71.4 21.3 38–98
14 14 124.1 42.1 95–218
15 15 136.8 43.7 64–245
16 16 191.2 59.7 27–285
17 20 252.6 98.5 140–573
18 4 289 150 70–410
19 14 324.5 65.2 241–470
20 3 380 39 355–425
Source: Data from ref. 63.
to 1861 EA procedures, of whom 38 had cytogenetic abnormalities, 3 were lost to follow-up, and
1820 were cytogenetically normal. Forty-two, or 2.3%, were lost or delivered before 28 weeks. Clubfoot
was seen in 0.2% of CVS patients; in EA patients, it was seen in 1.2% of 11- to 12-weekprocedure
offspring, 0.8% of 13-week offspring, and 0.2% of 14-week offspring for a relative risk of
EA versus CVS of 4.1 (1.17–14.6). The authors concluded that, in general, CVS is the preferred
prenatal diagnostic procedure between 12 and 14 weeks (81)
Specimen Requirements
The volume of amniotic fluid obtained for prenatal diagnosis varies with the stage of gestation,
with 15–20 mL conventionally removed by mid-trimester amniocentesis practitioners. In one report,
data from several small studies was pooled and the volume of amniotic fluid for weeks 10–20 was
calculated (63) (see Table 9). At gestations under 15 weeks, many practitioners have adopted the
practice of removing 1 mL per week of gestation, and others have found excellent culture success rate
and turnaround time with less fluid removed. For example, one group withdrew 4–12 mL in gestations
of 9–14 weeks and obtained a 100% culture success rate in 222 specimens (82), whereas others
withdrew 5–8 mL in pregnancies of 10 weeks, 4 days to 13 weeks, 6 days for an overall culture
success rate of 99.7% (79). It has been observed that the total cell numbers rise exponentially from 8
to 18 weeks’ gestation, but the number of viable cells increases only slightly during that time (73).
This probably explains the comparable culture success rate of EA compared to mid-trimester amniocentesis.
Chorionic Villus Sampling
Associated Risks, Limitations, Benefits, Turn-Around Time
Risks associated with CVS have extensively been studied. Perhaps the issue receiving the most
attention in the past few years was raised by Boyd et al. involving 1 case (83) and then more extensively
by Firth et al. (84), who reported 5 babies with severe limb abnormalities out of 289 pregnancies in
which TA CVS had been performed at 56–66 days’ gestation. Four of these had oromandibular-limb
hypogenesis syndrome. They hypothesized that CVS undertaken up to 66 days’ gestation might be
associated with an increase in the risk of oromandibular-limb hypogenesis syndrome and other limb
reduction defects. This report generated many others, with mixed conclusions.
A flurry of letters to the editor of Lancet in 1991 followed Firth’s report. Reporting evidence to
support the association between CVS and limb reduction defects were Mastroiacovo and Hsieh