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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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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 is present.
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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.
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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 "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 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 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.
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From Franklin Tessler, MD, CM (FTessler@aol.com)
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 sonogram.
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.
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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.
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(The nuchal fold is on the back of the neck.)
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.
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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.
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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.
Some estimates:
"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.)
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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 duplication.
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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)
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From Robbrenner@aol.com (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.
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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.
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From Robbrenner@aol.com (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.
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