This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (firstname.lastname@example.org) with numerous contributions by others.
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 Robbrenner@aol.com (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 (FTessler@aol.com)
[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
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 using color.)
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
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.