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misc.kids FAQ on Ultrasound
Last-Modified: February 16, 1995
Misc.kids Frequently Asked Questions
Collection maintained by: Lynn Gazis-Sax )
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
AFP Screen and the Triple Screen. Please try to be as concise as possible,
as these FAQ files tend to be quite long as it is. And, unless
otherwise requested, your name and e-mail address will remain in the
file, so that interested readers may follow-up directly for more
For a list of other FAQ topics, ftp to the pub/usenet/misc-kids directory
of rtfm.mit.edu or tune in to misc.kids.info.
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.
[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.]
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.
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.
1. What are the different kinds of ultrasound and what can they detect?
Ultrasound is high frequency sound waves which are used to visualize the
fetus in utero. It works in a fashion similar to sonar.
Ultrasound is used at a variety of different points in pregnancy to detect
a variety of different things. Uses of ultrasound include: 1) to guide
instruments for prenatal diagnosis (as, for example, the needle used in
amniocentesis), 2) to confirm pregnancy, 3) to locate the baby (useful in
ruling out ectopic pregnancy), 4) pregnancy dating, 5) to determine
whether there is more than one baby, 6) to check the baby's growth, 7) to
evaluate movement, tone, and breathing, 8) to identify sex (not as
reliably as amniocentesis - don't paint the nursery based on this
information), 9) to assess the amount of amniotic fluid, 10) as an adjunct
to cervical cerclage or suture, 11) to look for molar pregnancies, 12) to
determine the structure and position of the placenta (particularly useful
if placenta previa is suspected), 13) to determine the cause of bleeding,
14) for fetal surgery, and 15) to confirm fetal death. Details on all of
these uses can be found in _Prenatal Tests_ by Robin Blatt. Some places
(e.g. India) are considering outlawing informing the parents of the sex of
the child based on ultrasound, because of the tendency for female children
to be aborted.
The disabilities which ultrasound can detect are those which show up in
the picture of the fetus, for example, anencephaly (by the twelfth week of
pregnancy), spina bifida (by the twentieth week), disorders of the
skeleton, central nervous system, heart, kidneys, or urinary tract.
Ultrasound does *not* detect the severity of spina bifida, only whether it
From (Robert Brenner MD):
Ultrasound can detect numerous structural defects. The list is too long
to mention. ULTRASOUND CANNOT DETECT DOWN SYNDROME WITH ANY DEGREE OF
ACCURACY. The American College of OBGYN states that routine ultrasound is
not cost effective and does not influence neonatal outcome. Therefore,
ultrasound is recommended only for indicated reasons such as bleeding,
inaccurate dates, large or small for dates, family history or past history
of structural birth defects that can be diagnosed by ultrasound, elevated
AFP, abnormal triple screen, and for guidance at the time of
amniocentesis. Late in pregnancy ultrasound is used to determine fetal
well being, the amount of amniotic fluid, the position of the fetus, and
to get an estimate of the size of the fetus. Ultrasound is routinely used
to follow fetal growth in multiple gestations as well as fetuses who are
small for gestational age.
From Franklin Tessler, MD, CM )
[LGS: The part in square brackets is a correction of a paragraph which used
to be in this FAQ - I removed the paragraph, but kept the correction, because
I have seen posts asking about the meaning of the term "Level II ultrasound."]
[The distinction between "Level I" and "Level II" ultrasound has nothing to
do with the type of image produced. Rather, these terms have come to refer to
the level of detail of an OB sonogram; that is, a routine "dating" sonogram
would be considered Level I and a specialized or "targeted" exam would be
considered Level II.
In actual fact, professional ultrasound organizations such as the American
Institute for Ultrasound in Medicine do *not* recognize the validity of these
terms. Furthermore, all imaging ultrasound nowadays is "real time," and most
Doppler ultrasound used to characterize blood flow during pregnancy is
pulsed, rather than continuous. It is also untrue that ultrasound is only
capable of detecting structural problems: for example, some cardiac
arrhythmias (abnormal heart rhythms) can be detected sonographically.]
Here is my suggested response:
Ultrasound uses high-frequency sound waves to produce two-dimensional
pictures of the body, including the fetus and its environment.
(Three-dimensional ultrasound is being investigated.) Most of the time, these
images are produced by placing a hand-held device called a transducer against
the skin. (Sometimes, even clearer images can be produced by inserting a
special transducer into a body cavity such as the vagina.) Most modern
ultrasound equipment is capable of depicting moving structures such as the
baby's heart, hence the term "real-time."
Doppler ultrasound also uses sound waves, but instead of producing a picture,
it shows the speed and direction of blood flowing through vessels. (A newer
variant called color Doppler ultrasound depicts blood flow in pictorial form
In the first trimester, ultrasound is most often used to determine whether a
pregnancy is properly located within the uterus or is located in an abnormal
position (ectopic pregnancy), or to confirm suspected miscarriage. This is
also the most accurate time for dating pregnancy. First trimester ultrasound
is often done using a vaginal approach.
In the second trimester, ultrasound can be used to answer questions about and
its surroundings, for example: How many babies are there? Where is the
placenta located? Is the amount of amniotic fluid normal? How far along is
the pregnancy? Ultrasound also is invaluable to guide interventional
procedures such as amniocentesis.
2. What can ultrasound not detect?
From Franklin Tessler, MD, CM )
As far as detecting fetal abnormalities goes, several points need to be made:
a) Not every problem can be diagnosed with ultrasound -- conditions which do
not manifest as a structural or gross functional abnormality (such as a very
abnormal heart beat) may be missed.
b) Not every problem which can be detected will be diagnosed. For example,
the basic ultrasound exam (for which there are published guidelines) does not
include counting the baby's fingers and toes, even though it is possible to
do so should it be necessary.
c) The sensitivity of an ultrasound exam depends on a number of factors, such
as the size and position of the fetus, the body habitus of the mother, the
type of equipment used, and, most importantly, the skill and experience of
the operator. Concerned parents-to-be may want to inquire politely about the
training and experience of the person performing or interpreting their
d) Some problems (such as anencephaly) are more readily diagnosed than others
(such as cleft palate).
In the third trimester, ultrasound can be used to detect problems that may
affect planning of delivery, such as intrauterine growth retardation (IUGR).
As mentioned elsewhere, dating during this stage of pregnancy tends to be
less accurate because biological variability is greater.
From Dr. T. Reynolds:
Amazing claims are being made about nuchal fold thickness measurement as
a screening technique for Down's but this technique is being performed in
highly specialised teaching centres and there is as yet no evidence that the
test could be carried out in 'lower-tech' local hospitals.
(The nuchal fold is on the back of the neck.)
3. How accurate is ultrasound, and what are possible sources of error?
The accuracy of ultrasound for dating a pregnancy depends on at what point
during the pregnancy the ultrasound is taken. Pregnancy dating is most
accurate during the first half of pregnancy. Measurement of the sac at
five to seven weeks gives an accuracy of plus or minus ten days.
Measuring the crown-rump length gives an accuracy of plus or minus three
days at seven weeks; this test can be used from the seventh to the
fourteenth week. Between fourteen and twenty-six weeks, the measurement
of the biparietal diameter of the baby's head, the femur length, and the
head and abdominal circumference is used; the accuracy is plus or minus
seven to ten days. Later in pregnancy, the accuracy declines, and may be
plus or minus twenty-one days.
From Dr. T. Reynolds:
The reason for this is that different babies grow at different rates and that
all measurements are subject to inacccuracy because the object being measured
is not linear (e.g. a babies head is not a sphere, it is an ellipsoid, so
it is possible to get different slightly measurements depending on what
position the baby lies in.
I haven't found many estimates of exactly how accurate ultrasound is at
detecting disabilities, but there are both false negatives and false
positives. The accuracy will vary depending on the experience of the
person doing the ultrasound. The accuracy also varies with which
condition is being detected. For Down Syndrome, it is very low. For
anencephaly, on the other hand, it is highly effective.
"The use of routine ultrasound, including a four-chamber view of the
heart, can lead to the diagnosis of approximately 50 percent of major
cardiac, kidney, and bladder abnormalities that would not be detected by
maternal serum alpha-fetoprotein screening. When targetted ultrasound
examination is performed by skilled ultrasonographers to detect
malformations suspected on the basis of the history or the screening
ultrasonogram, the sensitivity and specificity of this procedure are
greater than 90 percent." (NEJM, 1/14/93, Prenatal Diagnosis) It is
estimated that ultrasound can detect 81% of ectopic pregnancies (Brit
Journal of Obst and Gyn, Dec 1988, Vol 95, pp 1253-1256). Ultrasound is
most effective for gross structural abnormalities. It is highly effective
for anencephaly. (Medical Intelligence. Chervenak et al. Advances in the
Diagnosis of fetal defects.)
From Dr. T. Reynolds
For spina bifida there is definitely evidence (but I can't remember where I saw
it) that diagnostic accuracy is improved by having the AFP test: i.e. a high
AFP result concentrates the mind of the ultrasonographer and they look for and
often spot smaller neural tube defects. It is for this reason that some centres
continued screening for spina bifida using AFP even when ultrasound arrived and
certain quarters called for an end to the blood test because it was unnecessary
4. What are the risks of ultrasound?
This question turns out to be controversial. Some of the books which I
consulted reassured that ultrasound has been used for decades with little
risk, and that, while more studies could be done, the studies which have
been done confirm its safety. "Although the effects of ultrasound are
still being studied, no harmful effects to either the mother or the baby
have been found in over 20 years of use. The long-term risks of
ultrasound, if any, are unknown, but there are many benefits." (ACOG)
Others warn that it is insufficiently tested, and make comparisons with
X-rays and DES, which were once considered safe. The main area of debate
is whether ultrasound is being used too frequently in a routine fashion in
healthy pregnancies, without thorough enough testing. There may be some
association between ultrasound and low birth weight (Blatt). (Altho' this
is difficult to prove because the opposite assertion is known to be true:
i.e. if there is IUGR (intra-uterine growth retardation) US will be
performed more regularly to assess the progress of the baby.)
Some people express concern about the heat and cavitation (bubbling in
the cells). Others wonder whether routine ultrasound is cost effective,
or whether the money involved would be more effectively spent elsewhere.
A Consensus Development Conference of the National Institutes of Health
(NIH) was convened to consider the use of ultrasound in pregnancy, and
concluded that "Diagnostic ultrasound is considered to be a low-risk
procedure. However, routine use of ultrasound in pregnancy should be
discouraged." (Blatt) They recommended ultrasound only for twenty-eight
specific instances. (The use of diagnostic ultrasound in pregnancy.
Washington, DC. Government Printing Office, 1984.) In contrast, Germany,
France, and the UK have adopted a policy of ultrasound for all
pregnancies. (NEJM, 1/14/93, Prenatal Diagnosis)
From (Robert Brenner MD):
The theoretical risk of fetal exposure to sound waves has never been shown
to cause any fetal damage. The biggest risk of ultrasound is
overinterpretation or missed diagnosis.
From: (Suzanne Amador)
Suggested addition to the Ultrasound FAQ section on Ultrasound Safety:
Here are a list of references from the medical literature on human
population studies which examine the risks and benefits of ultrasound
screening during pregnancy. These references show that there is
presently very good evidence that no short or long-term effects result
from prenatal exposure to ultrasound. However, ultrasound exposure at
high intensities and long exposure times can cause problems in
laboratory animals. (see refs. (1, 2), for example.) Most of these
problems appear to be due to the heating which can result from long,
high intensity ultrasound. Thus, these human population studies have
been conducted to see whether such effects occur at the exposure times and
intensities actually used in clinical prenatal ultrasound. None of the
studies listed below specifically address vaginal ultrasound exams or
long-term exposure to Doppler ultrasound.
Several large human population studies have been performed in which
thousands to tens of thousands of women in low-risk pregnancies are
assigned randomly to either control groups or routinely-exposed groups.
The control groups are assigned to receive no routine ultrasound exams,
while the routinely-exposed groups are routinely examined, regardless of
need. Women in either group are given ultrasound exams if a problem
arises which makes the exam medically necessary, but no women are moved
between groups after their initial assignments. (This ensures that
women who develop problems during pregnancy aren't steadily moved from
the control to the routinely-exposed group. Were this the case, it
would not be surprising that the incidence of problems would increase in
the routinely-exposed group.) All but one of the large studies show no
evidence of harm, even for the large populations studied. (see refs (4,
7) and references therein.) The study by Newnham et al. (ref. 6) did
see an increase in the percentage of low-birthweight babies in an
intensively-examined group versus a control group receiving a very low
average number of exams; apart from this difference, the two groups had
identical pregnancy outcomes. However, similarly conducted studies
(refs. 4, 7) examined much larger populations, and found no problems.
Newnham also notes that the average difference in birthweights between
the two groups was very small, so that the finding could have been an
accident due to small numbers of cases.
Some studies don't bear out advantages from routine ultrasound for
low-risk pregnancies (see refs. 4,6). These studies are always designed
to exclude women who already have indications that their pregnancies
will present problems, such as unexplained bleeding, the assumption
being that ultrasound exams offer a net benefit for pregnancies with
known risk factors. However, some physicians think that these studies
underestimate the benefits of routine ultrasound; objections such as
these are aired in the correspondence following (refs. 4 and 6).
Most of the population studies listed below only follow women through
pregnancy and birth, but some are of long enough term to study children
past infancy (refs. 5,8,9) These are again reassuring, although the
numbers of children followed is much smaller than in the studies of
infants immediately after birth. One study (ref. 3) received wide media
coverage because it purported to show an association between hearing
loss and ultrasound exams. The authors studied a condition, delayed
speech, in a group of Canadian children. This condition is not known to
be caused by physiological problems, and may be a problem with
psychological origins. In their study, they compared a group of
children with this condition with a group of children of the same size
without delayed speech. This matching of the two groups was performed
after the fact, rather than by following two groups of children from
birth and watching them potentially develop this problem. The numbers
of children studied were very small (under one hundred, compared to the
much larger numbers examined in every other study mentioned here), and
the researchers didn't control for numerous other factors. Other major
problems in this study are detailed in the correspondence following the
article, in which the authors actually state that they do not consider
their work to show a link between ultrasound and delayed speech.
References on obstetrical diagnostic ultrasound and studies of
1. Barnett, G.R. ter Haar, M.C. Ziskin, W.L. Nyborg, K. Maeda, J. Bang,
"Current status of research on biophysical effects of ultrasound,"
Ultrasound in Medicine and Biology, vol. 20 (1994) pp. 205-218.
2. Bioeffects and Safety of Diagnostic Ultrasound, American Institute
of Ultrasound in Medicine, Rockville, Maryland, 1993.
3. Campbell, R.W. Elford, R.F. Brant, "Case-control study of prenatal
ultrasonography exposure in children with delayed speech," Canadian
Medical Association Journal, vol. 149 (Nov. 15, 1993), pp. 1435-40; Also
read the related correspondence in the same journal, vol. 150 (March 1,
1994), pp. 647-9.
4. Ewigman, J.P. Crane, F.D. Frigoletto, "Effect of Prenatal Ultrasound
Screening on perinatal outcome," New England Journal of Medicine, Vol.
329, pp. 821-7, 1993.
5. Lyons, C. Dyke, and M. Toms, "In utero exposure to diagnostic
ultrasound: a six year followup," Radiology, vol. 166 (1988) pp. 687-690.
6. Newnham, John P., Sharon F. Evans, Con A. Michael, Fiona J.
Stanley, and Louis I. Landau, "Effects of frequent ultrasound during
pregnancy: a randomised controlled trial," The Lancet, Vol. 342,
October 9, 1993, pp. 887-891; see also related correspondence in the
same journal, Nov. 27, 1993, pp. 1359-61 and Jan. 15, 1994, p. 178.
7. Saarri-Kemppainen, O. Karjalainen, P. Ylostalo, O.P. Heinonen,
"Ultrasound screening and perinatal mortality: controlled trial of
systematic one-stage screening in pregnancy. The Helsinki Ultrasound
Trial," The Lancet, vol. 336 (1990), pp. 387- 391.
8. Scheidt, F. Stanley, D.A. Bryla, "One year follow-up of infants
exposed to ultrasound in utero," American Journal of Obstetrics and
Gynecology, vol. 131 (1978), pp. 743- 748.
9. Stark, M. Orleans, A.D. Havercamp, "Short and long-term risks after
exposure to diagnostic ultrasound in utero." Obstetrics and Gynecology,
vol. 63 (1984) pp. 194- 200.
5. Do you really have to have a full bladder for an ultrasound?
Women getting an ultrasound are encouraged to drink several glasses of
water an hour before the exam and not go to the bathroom until after the
exam. The full bladder helps the doctor locate the pelvic organs and get
a clearer and more accurate picture. However, this advice only applies
for some ultrasounds, depending on the kind of probe used and the point
in pregnancy when the ultrasound is done. This is why different women
report getting different advice from their doctors about whether a full
bladder is required.
From (Robert Brenner MD):
The two types of ultrasound are abdominal and vaginal ultrasound. Vaginal
ultrasound is the most accurate up to 12 weeks gestation and does not
require a full bladder. Abdominal ultrasound requires a full bladder up
to about 14-16 weeks.
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.
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