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Archive-name: misc-kids/pregnancy/screening/amniocentesis
Posting-Frequency: monthly Last-Modified: February 16, 1995 See reader questions & answers on this topic! - Help others by sharing your knowledge
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Misc.kids Frequently Asked Questions
Amniocentesis
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Collection maintained by: Lynn Gazis-Sax (gazissax@netcom.com)
To contribute to this collection, please send e-mail to the address
given above, and ask me to add your comments to the FAQ file on
Amniocentesis. Please try to be as concise as possible,
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Copyright 1995, Lynn Gazis-Sax. Use and copying of this information are
permitted as long as (1) no fees or compensation are charged for
use, copies or access to this information, and (2) this copyright
notice is included intact.
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[NOTE: this is information collected from many sources and while I
have strived to be accurate and complete, I cannot guarantee that I
have succeeded. This is not medical advice. For that, see your
doctor or other health care provider.]
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Acknowledgements:
Many people helped with the prenatal testing FAQs by advising about
the best way to structure them, by contributing stories and information,
or by reviewing versions of the FAQs. A list of acknowledgements can
be found in the Prenatal Tests: Overview FAQ.
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Note on language: When I first posted the questions for the prenatal testing
FAQs, I used the term "birth defects" (except for question 7 of the Prenatal
Testing Overview FAQ). Since I have been advised that this term may be
offensive to people in the disabled community, I changed the wording of the
final FAQs to use the word "disability," but most replies still reflect the
original wording of the questions.
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V. Amniocentesis
1. What is amniocentesis?
Amniocentesis, or amnio, is a prenatal test in which a needle is inserted
into a woman's abdomen to remove a portion of the amniotic fluid. This is
usually done in conjunction with an ultrasound test so that the doctor can
see where he is putting the needle to avoid harming the fetus. By
performing biochemical tests and by examining the fetal cells in this
amniotic fluid, it is possible to detect certain disabilities.
2. What can it detect?
One of the main uses of amniocentesis is to detect chromosomal
abnormalities. With amniocentesis, it is possible to reconstruct the
chariotype of an individual, i.e. map his or her chromosomes. This allows
the detection of trisomies (extra chromosomes), monosomies (missing
chromosomes), and other structural defects in the chromosomes.
Most people have 46 chromosomes, in 23 pairs, but some have an extra copy
of one chromosome, called a trisomy because there are three of one
chromosome pair. This extra chromosome can lead to a variety of
abnormalities. The most common trisomy is called Down syndrome, a trisomy
of chromosome pair 21, and it leads to mental retardation and various
physical problems. Other common trisomies are trisomies 13 and 18 (which
generally cause a baby to die shortly after birth) and sex chromosome
trisomies. Examining the chromosomes also allows the sex to be
determined, which may be of particular interest to women who are carriers
of sex-linked disorders, such as hemophilia or Duchenne muscular
dystrophy.
In addition to examining the fetal cells for chromosome abnormalities, the
amniotic fluid can be tested for levels of AFAFP, acetylcholinesterase
(AChe), and hemoglobin F. It is possible to detect neural tube defects,
including anencephaly, spina bifida, and meningomyelocele (though the use
of amniocentesis to detect neural tube defects has been mostly superceded
by a combination of the AFP test and high resolution detailed ultrasound).
It is also possible to detect about 70 metabolic disorders. The tests for
metabolic disorders, however, are only done if family history warrants,
and will not be done for women being referred for amnio due to age or
results on the AFP test or Down's screen. If there is no history of
genetic disease in the family, a genetic analysis will not be performed,
only a chromosomal analysis. Chromosomal analysis is performed at most
large hospitals and some private labs. Genetic analysis is only performed
in a few labs in the country and is significantly more expensive. Tay
Sachs, which is common among Eastern European Ashkenazi Jews and French
Canadians, can be detected by amniocentesis. One of my sources (Blatt)
states that amnio can be used for certain experimental DNA studies,
detecting cystic fibrosis, sickle cell anemia, thalassemia, and other
blood disorders with varying degrees of accuracy. Another source (Scher
and Dix) says that sickle cell anemia and thalassemia cannot be detected
by amniocentesis, but can be detected by an experimental procedure called
fetoscopy.
Clarification by Dr. T. Reynolds:
It is possible to collect blood samples from the fetus by amnio by guiding the
needle into an umbilical vessel. We don't do it in my hospital because our
ethnic mix doesn't warant the development of the expertise.
Since several different prenatal tests are used to detect the same disabilities,
information on the disabilities which amnio and other tests
detect is included in the Prenatal Testing overview FAQ.
3. What can an amnio not detect?
Amniocentesis will not guarantee you a normal child. It cannot detect
most non-chromosomal genetic defects, nor can it detect defects of body
structure, such as harelip, cleft palate, congenital heart disease,
hypospadias, pyloric stenosis, clubfoot or congenital hip dislocation.
Defects caused by exposure to toxic substances will also not be detected
by amnio.
4. What are the risks of amnio?
The main risk of amnio is that it may increase the chance of miscarriage.
Different sources disagree on how much. One of my sources (Scher and Dix)
says that controlled studies have shown no difference in miscarriage rate
among women who undergo amniocentesis and those who do not; the rate is
0.5% for both. Another source (ACOG) says that the risk of miscarriage
for pregnancies from 16 weeks on is 3% normally, and that amnio increases
that risk by 0.5%. Blatt says that the most common figure given is 0.5%,
but that some sources cite 1% to 1.5%, and Rothman gives a 3 in 1000 risk
that an amnio will result in miscarriage.
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From Dr. T. Reynolds:
Some of the dispute is because different studies defined miscarriage
differently: in some only fetal loss within a few weeks of the amnio was
considered whereas in other even still birth was blamed on the amnio. Probably
the studies counting fetal death within 4 weeks of amnio give the best estimate.
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From Rob Brenner, MD:
The pregnancy loss rate after amniocentesis is 1/270. This is the same as
the backround loss rate at 16 weeks. Even though there is no statistical
increase in pregnancy loss after amniocentesis, it is an invasive procedure
and patients should be appraised of a potential loss of the pregnancy as a
result of infection, bleeding, or rupture of membranes. The incidence of
fetal injury is negligible.
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After the test, you may experience cramping, fever and chills, vaginal
bleeding or leaking amniotic fluid. Call the doctor if these are severe
or persist.
5. What kinds of error are possible in amnio?
Amniocentesis is generally accurate. The most common error is a cell
culture failure, in which case the amniocentesis may need to be repeated.
This happens about 2% of the time (Scher and Dix). Other possible sources
of error are maternal cell contamination (so that cells from the mother
are examined instead of cells from the fetus), artifacts of the testing
process (such as pseudo-mosaicism), or mislabelling of the sample. (In
order to avoid these errors, labs will check a certain number of cells
before diagnosing an abnormality.) Better results are obtained from more
experienced labs.
There are also some results which are inherently ambiguous, because we
don't really know the effect of the chromosomal abnormality detected.
Trisomy 21, or Down syndrome, is a well-defined syndrome (although, even
there, amniocentesis will not tell how severe a case of Down syndrome the
baby will have), but the results of sex chromosome trisomies are less well
known. Is XYY associated with more violent behavior or isn't it? Some
reports say yes, others no. The effects of XXX are also unknown. For
more discussion of these ambiguous diagnoses, see Barbara Katz Rothman's
book, _The Tentative Pregnancy_.
6. Under what circumstances is amnio usually given?
Amniocentesis is recommended when the risk of a disability detectable by amnio
is judged to be greater than the risk from amniocentesis. People for whom
amniocentesis is likely to be recommended include: women with an abnormal
result on the AFP or Down's screen which is not explained by ultrasound
(indicating, depending on the result, either increased risk of Down
syndrome or increased risk of neural tube defects), women with a previous
child with a disability which amnio can detect (in whom it is unreasonable
to do a Down's screen because their prior risk is so high that the Down's
screen should not alter the Obstetrician's action), women who have had
three or more miscarriages (this one is questionable because amnio can
cause miscarriage and in many cases of recurrent abortion a cause is not
known), carriers for conditions (such as Tay Sachs) which can be detected,
carriers of sex-linked conditions such as hemophilia, pregnancies in which
one of the parents is known to have rearranged chromosomes, women with
ultrasound results showing a fetal anomaly compatible with a chromosomal
abnormality, and advanced age.
The most common reasons for having an amnio recommended are AFP results
and age. The reason for giving amnios to older women is that chromosomal
abnormalities (mainly Down syndrome, but also other trisomies) are more
common in older women. When amniocentesis was first available, it was
recommended for women over 40. As it has become more available, the age
at which it is recommended has been lowered, and it is now often offered
to all pregnant women over the age of 35.
7. How early can an amnio be done?
Amniocentesis is most commonly done between the sixteenth and eighteenth
week of pregnancy (using the usual pregnancy dating system of counting the
weeks from the last period, rather than from conception). In some places,
it is experimentally offered as early as twelve weeks. After the amnio is
done, it takes two to four weeks to get the results.
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From Dr. T. Reynolds:
Some early results of 'early amnio' are not encouraging with miscarriage rates
of 3-5% as a result of early amnio compared with 0.5% for week 16.
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From Rob Brenner, MD:
Most amniocentesis is done at 16-18 weeks. Late amniocentesis is often
done in the third trimester to determine fetal lung maturity if early
delivery is indicated. Some centers are performing amniocentesis as early
as 12 weeks but the pregnancy loss rate is higher.
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8. What about chorionic villus sampling (CVS) as an alternative to amnio?
Chorionic villus sampling is an early surgical test in which part of the
chorion, the outer tissue of the sac surrounding the embryo, is removed
and analyzed. It is a newer test than amniocentesis, and is still
considered experimental. The chief advantage of CVS over amnio is that
the results are available much more quickly. CVS is done between the
ninth and twelfth week of pregnancy, and the results are available within
ten days.
As of 1989, ACOG no longer considers CVS experimental (March of Dimes)
The disadvantages of CVS are, first, that it does not detect neural tube
defects, as amnio does. Second, there may be some missed diagnoses due to
chorionic mosaicism. Third, there is a higher risk of miscarriage. There
are various estimates for this risk: 1% to amnio's 0.5% (ACOG), 1-5%
compared to .2% for amnio (Blatt), and others simply say that the safety
and long term effects of CVS are unknown. There is some evidence that CVS
may sometimes cause limb defects, but this evidence is inconclusive.
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From Robert Brenner, MD:
CVS(Chorionic villus sampling) is a procedure where a piece of the
placenta is aspirated into a plastic tube and cultured for chromosome
analysis. There are case reports of abnormal limb development following
CVS but this is thought to be avoided if CVS is done after 8 weeks
gestation. The pregnancy loss rate is higher than amniocentesis but the
backround loss at 10 weeks is higher also. The advantage of CVS is that a
diagnosis of chromosomal abnormality can be made earlier enabling a
patient to terminate her pregnancy by D&C rather than by prostaglandin
urea induction of labor as is done later in pregnancy (after
amniocentesis).
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More information on the limb defects: A study by the Centers for Disease
Control and Prevention, in 1994, found that infants whose mothers had
CVS had a 0.03% chance of missing or underdeveloped fingers or toes.
The normal risk is about 0.005%. Some researchers have said that this
study was poorly done and looked at too few births. Dr. Laird Jackson
of Thomas Jefferson University in Philadelphia said that he had followed
about 120,000 women who had CVS and found no such increase. A 1992
survey by WHO found miscarriage rates of from 1.2%-8.4% at different
medical centers worldwide (there is apparently a lot of variation in
miscarriage rates from one center to another), and a slight increase in
fetal limb defects, from 5.4 cases per 10,000 to 6 per 10,000. A study
published in the August, 1992 issue of the New England Journal of Medicine
found a 0.8% greater chance of miscarriage. More details can be found
in newspaper articles in the New York Times on March 12, 1994, July 15,
1994, and October 23, 1994, and in the article "Prenatal Diagnosis,"
in the New England Journal of Medicine, by Alton and DeCherney.
-----------------------------------------
From Dr. T. Reynolds:
One other interesting development which applies to some couples only is
polymerase chain reaction DNA testing: One case has been reported where
the husband had a Haemoglobinopathy (sickle-cell-type disorder) called
Haemoglobin Lepore-Boston (OK so its rather rare, but it's an example of what
can be done). A similar technique can be used to diagnose fetal sex from a
maternalblood sample if it is likely to be clinically important (e.g. for
avoidance of muscular dystrophy/other sex linked disorders). NOTE: some of
these techniques may only be available in big research centres.
-----------------------------------------
Sources:
Alton and DeCherney. "Prenatal Diagnosis." New England Journal of
Medicine. January 14, 1993.
The American College of Obstetricians and Gynecologists (abbreviated in
references as ACOG). Planning for Pregnancy, Birth, and Beyond. A
Dutton Book, May, 1992.
Blatt, Robin J.R. Prenatal Tests. Vintage Books. New York, August 1988.
The Boston Women's Health Collective. The New Our Bodies, Our Selves.
Simon and Schuster. New York, NY, 1992.
Rothman, Barbara Katz. The Tentative Pregnancy. Viking Penguin Inc. New
York, NY, 1986.
Scher, Jonathan, M.D., and Dix, Carol. Will My Baby Be Normal? How to
Make Sure. The Dial Press. New York. 1983.
Lynn Gazis-Sax
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