lotus



previous page: 2. What can ultrasound not detect?
  
page up: Pregnancy Screening FAQ
  
next page: 4. What are the risks of ultrasound?

3. How accurate is ultrasound, and what are possible sources of error?




Description

This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (gazissax@netcom.com) with numerous contributions by others.

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.

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.)


-----------------------------------------
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.
-----------------------------------------


 

Continue to:













TOP
previous page: 2. What can ultrasound not detect?
  
page up: Pregnancy Screening FAQ
  
next page: 4. What are the risks of ultrasound?