Expert: Catherine Beier MS CBE - 2/2/2010

Question
Hi Catherine,
 This website is very informative. I am however a bit confused and disappointed regarding certain bits of information.There are several experts on this website saying one thing, and several others making a totally opposite claim. I would like for things to be explained, as incorrect answers can mislead and misguide people who come here for advice. For example, as I am sure many other people,especially females, have noticed, some RELIABLE and PROFESSIONAL experts have mentioned that first trimester ultrasounds are NOTthe most accurate, as they can be off by as much as 2 weeks,even a MONTH!, and that the most accurate
ultrasounds are those done between 18-20 weeks. Other RELIABLE experts claim that first trimester ultrasounds are the MOST accurate, and are only off by as much as 3-5 days abdominally and 1-3 days vaginally. I don't wish to name people or to get
anyone here in trouble, but this conflicting information is truly troublesome, as both bits of knowledge cannot simultaneously be correct, and many people, especially women, might be receiving incorrect advice or information. Can you please clarify exactly how accurate EACH PHASE of ultrasound really is, in the first, second and third trimesters, and what the margin or error is in each trimester? As several experts have claimed, can first trimester ultrasounds truly be off by as much as 1 whole month? Why do some say that the most accurate time to take ultrasound is between 8-10 weeks, while other are saying it's between 18-20 weeks? Any clarification would be appreciated. Thanks and waiting for your response.

Answer
Hello Hilary,


For the purpose of your question, I'm going to assume that you're referring only to ultrasound use for the purpose of pregnancy dating.


Ultrasounds are more or less reliable at evaluating different measures at different weeks of pregnancy, so there's not a cut and dry answer over what time is best - it depends upon what the purpose of the ultrasound is and what factor(s) you're trying to rule out/in.


For example, if you are to only have a single ultrasound during your entire pregnancy, then the optimal time is between 18-20 weeks, to have the most reliable overall measures of fetal growth, well-being, development, placental position, etc.  


If you are instead concerned about a potentially deteriorating placenta, then third trimester ultrasound near term would be most appropriate.


Here is a breakdown of the most common uses of ultrasound during each trimester:

   * First Trimester:
         o Confirm viable pregnancy
         o Confirm heartbeat
         o Measure the crown-rump length or gestational age
         o Confirm molar or ectopic pregnancies
         o Assess abnormal gestation
   * Second Trimester:
         o Diagnose fetal malformation
               + Weeks 13-14 for characteristics of potential Down syndrome
               + Weeks 18-20 for congenital malformations
         o Structural abnormalities
         o Confirm multiples pregnancy
         o Confirm intrauterine death
         o Identify hydramnios or oligohydramnios – excessive or reduced levels of amniotic fluid
         o Evaluation of fetal well-being
   * Third Trimester:
         o Identify placental location
         o Confirm intrauterine death
         o Observe fetal presentation
         o Observe fetal movements
         o Identify uterine and pelvic abnormalities of the mother


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SIDEBAR

These factors can also explain why you're seeing conflicting answers here - because some questions might be asking the best time to have an ultrasound in general vs. the best time to have an ultrasound for dating purposes only, or the best time to have an ultrasound to rule out specific genetic defects, etc.

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ANOTHER SIDEBAR

The accuracy of all ultrasounds is also always dependent upon the skill of the technician and the quality of the equipment used.  If these factors are poor, then accuracy decreases.  Other factors, such as excess maternal weight and the position of the baby can also decrease its accuracy.

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The overall margin of error of ultrasound for gestational age and fetal growth has been found to be about +/-8% of the estimate at each trimester of pregnancy (Callen PW. The obstetric ultrasound examination. In: Callen PW editors. Ultrasonography in obstetrics and gynecology. 5th ed. Philadelphia: Saunders Elsevier; 2008;p. 3-25.)  In the first trimester, this would apply when the CRL (crown-rump length is measured).  In the second and third trimesters, this would apply when the head circumference/biparietal diameter and femur length are measured.  


However, +/-8% of the estimate means very different things at different weeks of pregnancy.


For example:

at 8 weeks this means a maximum margin of error of +/-5 days.
at 18 weeks, this means a maximum margin of error of +/- 10 days.
at 28 weeks, this means a maximum margin of error of +/- 16 days.


You can reference http://www.medscape.com/viewarticle/703501_3 for a more thorough breakdown of ultrasound use in general.



Now, to address ultrasound use for pregnancy dating purposes:


The most accurate time to have an ultrasound for the purposes of dating a pregnancy is transvaginal ultrasound between 8-10 weeks.  It has a maximum margin of error of +/-5 days, but +/- 1-3 days at most is typically seen.  This implies that the CRL is measured, not the gestational sac, as the latter is not a reliable measure of gestational age compared to the CRL.


The reason that early transvaginal ultrasound is the most accurate method of determining fetal age is because at this stage of development, all embryos divide and multiply at very, very similar rates - no genetic factors such as maternal diet/nutrition, genetics, individual growth rates, etc come into play until after the first trimester when the placenta takes over nourishment of the fetus rather than the yolk sac.


By 7 weeks gestation (typically corresponding to a blood HCG value of about 20,000), all embryos grow at a rate of 1mm per day, making it very easy to accurately date the pregnancy.  The fetal heart typically starts beating by a CRL of 5mm.  Here is a chart of the typical CRLs (in cm, not mm) at each early week:

6.1 Weeks: 0.4 cm
7.2 Weeks: 1.0 cm
8.0 Weeks: 1.6 cm
9.2 Weeks: 2.5 cm
9.9 Weeks: 3.0 cm
10.9 Weeks: 4.0 cm
12.1 Weeks: 5.5 cm
13.2 Weeks: 7.0 cm
14.0 Weeks: 8.0 cm


The common formula to determine menstrual age (weeks of pregnancy) = crown-rump length (in cm, not mm) + 6.5.


The CRL can be used through the 14th week of pregnancy, at which time the FL and HC take its place as the hallmarks on which dating measures are based - and at which time dating becomes less and less accurate.  More detailed information can be found at:  http://www.fpnotebook.com/OB/Rad/UltrsndPrgncyDtng.htm


If a later ultrasound is performed and dates vary les than +/-2 weeks from the original measurements as calculated by the CRL, then the due date should not be changed.  If the dates are more than 2 weeks discrepant, then the due date may be changed, but reasons for the disparity should also be investigated, which could include IUGR (intrauterine growth restricton), fluid abnormalities, gestational diabetes in the mother or other such conditions.



In summary, if someone here is saying that a first trimester ultrasound for dating purposes can be off a month in either direction, well, they're just plain wrong.  It's not possible.  The only explanation with a discrepancy that large is a medical complication such as intrauterine growth restriction, faulty equipment or gross technical error on the technician's part.  


If instead, they're saying that the best time for a single ultrasound is between 18-20 weeks, then they are in broad terms correct - this will give the best overall picture of how the fetus is developing and potential red flags to address.  It's not the most accurate in terms of dating, but is a better measure of overall fetal well-being.


I hope that helps to clarify your concerns.


Additional references and resources are as follows:

http://www.cochrane.org/reviews/en/ab000182.html
http://www.cochrane.org/reviews/en/ab001451.html
http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=search&db=pubmed&term=Paspulati%20[AU]%20AND%202004%20[DP]%20AND%20Radiol%20Clin%20North%20Am%20[TA]
http://www.ob-ultrasound.net/
http://www.brooksidepress.org/Products/Military_OBGYN/Ultrasound/1st_trimester_u...
http://www.guideline.gov/summary/summary.aspx?ss=15&doc_id=14180&nbr=7087



Best,
Catherine