932
UNIT SIX
Couples who have already had a child with a chromo-
some abnormality can have
chorionic villus sampling
(CVS),
which has the advantage of being performed as early as the
tenth week from conception, but carries a higher risk of being
followed by miscarriage than does amniocentesis. In CVS, a
physician samples chorionic villus cells through the cervix.
The basis of the test is that, theoretically, these cells are genet-
ically identical to fetal cells because they too descend from the
fertilized ovum. However, sometimes a mutation can occur in
a villus cell only, or a fetal cell only, creating a false positive
or false negative test result, respectively.
Fetal cell sorting is an experimental prenatal test that
separates the rare fetal cells that normally cross the placenta
and enter the woman’s circulation; then a karyotype is con-
structed from the sampled cells. It can be performed early
in pregnancy, but so far, it is too costly to be widely imple-
mented. Fetal cell sorting is safer than amniocentesis or CVS
sample are cultured and a karyotype constructed, which
reveals extra, missing, or translocated chromosomes or
smaller anomalies. However, additional tests on amniotic
uid or ± broblast DNA are necessary to detect mutations in
individual genes.
Chorionic
villi
Placenta
Catheter
Fetus
Cervix
Syringe
Vagina
(a)
Chorionic villus sampling
Uterus
Amniotic
membrane
(b)
Amniocentesis
Fetus
15–16
weeks
(c)
Fetal cell sorting
Rare fetal cells
FIGURE 24.13
Three ways to check a fetus’s chromosomes. (
a
) Chorionic villus sampling removes cells that would otherwise develop into the
placenta. Since these cells descended from the fertilized ovum, they should have the same chromosomes as the fetus. (
b
) Amniocentesis draws out
amniotic F
uid. ±etal cells shed into the F
uid are collected and their chromosomes examined. (
c
) Improved techniques for extracting and identifying
speci²
c cells allow researchers to detect fetal cells in a sample of blood from the woman.
FIGURE 24.14
In an ultrasound exam, sound waves are bounced
the embryo or fetus, and the pattern of deF
ected sound waves is
converted into an image. By thirteen weeks, the face can be discerned.
Ultrasound can now provide images that look three-dimensional.
Videos of embryos and fetuses are also possible.
The decision to have amniocentesis is based on risk. In the past, the
procedure carried about the same risk of being followed by miscar-
riage as the age-related risk of miscarriage for a thirty-²
ve-year-old
without a family history of chromosome abnormalities. This is why
the test was o³
ered only to women of “advanced maternal age”—
thirty-²
ve—for many years. Recently, however, the safety of the pro-
cedure has greatly improved, so that it is now justi²
ed for younger
pregnant women, too.
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