Examining the Evidence
Q Is chorionic villus sampling as safe as amniocentesis?
<huc>A</huc> Yes, according to this retrospective study, provided the practitioner has adequate training and experience. The authors analyzed 20 years of experience and found that the risk of pregnancy loss diminished over time for both test methods, but the reduction was more pronounced in women undergoing chorionic villus sampling (CVS). By the final epoch of study (1998–2003), there was no significant difference between the 2 methods.
Caughey and colleagues launched their study to explore the following questions: What is the rate of pregnancy loss in women who undergo CVS and amniocentesis, compared with those who do not? Has the higher rate of pregnancy loss associated with CVS changed over the past 20 years?
That invasive diagnostic procedures have a learning curve is no surprise.1 It generally follows that the more efficient an operator becomes at a given invasive test, the lower the rate of complications. The clinical question facing us after this study is how valid the comparison is between the 2 procedures, even in the final 5-year epoch.
Study design is laudable
Caughey and colleagues did an admirable job of compiling data on nearly 10,000 CVS tests and 31,000 amniocentesis procedures and their associated clinical outcomes. The fact that this investigation was based at a single center with good follow-up is a definite strength. Also laudable is the attempt to control for background loss rate by adjusting for gestational age at the time of sampling in multivariable analysis, as well as the identification of a control group that underwent neither test.
The investigators also restrain themselves from extrapolating their conclusions or overstating their findings given the nonrandomized nature of the study.
Route of CVS was not specified
Unfortunately, we are not told whether the CVS procedures were performed transcervically, transabdominally, or using both approaches (as is common in many modern programs). Earlier reports involving transcervical sampling found a clear relationship between proximity of the placenta and cervix, as well as uterine position, and the risk of pregnancy loss.2 That is one reason centers began to choose the sampling route based largely on placental location.3,4 If the sampling route was individualized in this study, then the observations can be generalized to programs using a similar approach.
Other potential weaknesses (also cited by the authors) include limited demographic data among the entire population for habits or preconditions that might confound pregnancy loss, such as tobacco use and socioeconomic status. The mixture of experienced clinicians and trainees is another concern, although the authors claim they were equally distributed over the time frame.
More definitive answers are needed
Although this study will be useful in counseling patients who are considering invasive testing, it fails to answer the question of safety definitively. Such an answer requires randomization prior to CVS or the limiting of both procedures to the same gestational age range.
In the most recent head-to-head comparisons at similar gestational ages (11–14 weeks), CVS appears to be safer than amniocentesis.5,6 The same cannot be said for testing later in the mid-trimester, when amniocentesis is usually performed and has a well-established track record for safety.
1. Silver RK, MacGregor SN, Sholl JS, Hobart ED, Waldee JK. An evaluation of the chorionic villus sampling learning curve. Am J Obstet Gynecol. 1990;163:917-922.
2. Rhoads GG, Jackson LG, Schlesselman SE, et al. The safety and efficacy of chorionic villus sampling for early prenatal diagnosis of cytogenetic abnormalities. N Engl J Med. 1989;320:609-617.
3. Jackson LG, Zachary JM, Fowler SE, et al. A randomized comparison of transcervical and transabdominal chorionic villus sampling. N Engl J Med. 1992;327:594-598.
4. Silver RK, MacGregor SN, Muhlbach LH, Kambich MP, Ragin A. A comparison of pregnancy loss between transcervical and transabdominal chorionic villus sampling. Obstet Gynecol. 1994;83:657-660.
5. Sundberg K, Bang J, Smidt-Jensen S, et al. Randomised study of the risk of fetal loss related to early amniocentesis versus chorionic villus sampling. Lancet. 1997;350:697-703.
6. Philip J, Silver RK, Wilson RD, for the EATA Study Group. Late first trimester prenatal diagnosis: results of an international randomized trial. Obstet Gynecol. 2004;103:1164-1173.