First Trimester Anatomy
Abstract & Commentary
By John C. Hobbins, MD
Professor, Department of Obstetrics and Gynecology,
University of Colorado School of Medicine, Aurora
Dr. Hobbins reports no financial relationships relevant to this field of study.
Synopsis: A meta-analysis of 19 studies involving first trimester screening for fetal anomalies has shown that a majority of major structural abnormalities can be diagnosed between 11 and 14 weeks and that even cardiac abnormalities can be identified with ultrasound with reasonable efficiency.
Source: Rossi AC, Prefumo F. Accuracy of ultrasonography at 11-14 weeks of gestation for the detection of structural anomalies: A systematic review. Obstet Gynecol 2013;122:1160-1167.
As mentioned in previous alerts, standard screening protocols for fetal aneuploidies have been revamped to include, and actually feature, cell-free DNA (cf DNA). Most "combined" screening protocols had been built around first trimester assessment of nuchal translucency (NT). However, for high-risk patients, the emergence of cfDNA has rendered most previous screening combinations superfluous. The original NT is the possible exception — since it remains a useful adjunctive tool for aneuploidies not detected by cfDNA, as well as for many cardiac abnormalities and multiple anomaly syndromes. So, rather than abandon first trimester ultrasound in today's restructured screening protocols, some have expanded the NT exam to include first trimester fetal anatomy surveys.
Two authors from Italy scanned the literature for studies that explore the efficacy of a first trimester anatomy survey to detect major anomalies. Of the more than 1000 articles evaluated, only 19 lived up to their stringent statistical rules of inclusion. Together these studies involved 78,000 patients who had ultrasound examinations between 11 and 14 weeks. The major anomaly rate in this mixed-risk population was 12 per 1000 and the overall detection rate was 54%. The best detection rate involved abnormalities of the fetal neck (92%), abdomen (88%), brain and spine (51%), and fetal heart (48%).
Data involving the fetal heart were particularly interesting. These were the most common abnormalities encountered in the meta-analysis — noted in 418 patients, 201 of which were detected. About half of the heart abnormalities were identified by echocardiogram, while the other half were detected by the complete anatomic survey alone. However, the detection rate was far superior with the concentrated echocardiographic approach than with the standard fetal survey alone (50% vs 13%). Doppler investigation did not increase the detection rate.
Although 89% of the major anomalies were isolated, the detection rates were much higher when multiple anomalies were present (60%) than if isolated (40%). Also, having risk factors (such as maternal age, family history of cardiac or other anomalies, or exposure to potential teratogens) increased the chances of detection (60% vs 50%), but only 18% of the anomalies in the study came from patients in the high-risk category. If one combined transvaginal sonography (TVS) and trans-
abdominal sonography (TAS), the overall detection rate was 62%, with 51% by TAS alone and 34% by TVS (which was a surprise to me).
In the past, most screening strategies had been directed toward detecting aneuploidies and, in particular, trisomy 21, simply because of its association with advanced maternal age. However, the rate of aneuploidy in the overall population is small potatoes compared with the rate of major structural abnormalities, which complicate about 2% of all pregnancies. By switching the thrust of today's investigation to the fetal anatomy, not only will most anomalies be detected, but also most of the aneuploidies — which tend to be associated with structural abnormalities.
The major problem with screening for anomalies between 11 and 14 weeks is the inability to identify some anomalies that take time to develop. For example, agenesis of the corpus callosum and some causes of ventriculomegaly, along with abnormalities involving the posterior fossa, do not become apparent until after 20 weeks. Also, in many cases the small size of the fetal organs makes imaging difficult, especially if using TAS in obese patients.
In the interest of cost-effectiveness, it is often recommended that fetal surveys be postponed until the second trimester. However, this recommendation would only be appropriate if we were limited to one ultrasound examination per pregnancy. If the emphasis were more on efficiency and convenience, screening for anomalies would not be limited to an "either or" choice. In fact, few would argue against first trimester surveys being done in high-risk patients, but, as demonstrated in this study, only 18% percent of the anomalies in the total population came from patients with high-risk factors.
A major reason to move any screening protocol into the first trimester is to meet our patients' requests for earlier information that will either allay their anxieties or, if an anomaly is found, can initiate further testing to allow them to weigh their options at that time. Unfortunately, just because patients want this does not necessarily mean that in a, now, cost-conscious health care atmosphere, this initiative will fly with third-party payers.
The effectiveness of a first trimester anatomy search was well demonstrated in this study, but what about the cost? This could be based in part on the effort generated to accomplish this task. With this in mind, we initiated a small pilot study to determine how often in the first trimester we could clear the anatomy using the standard American Institute of Ultrasound in Medicine (AIUM) protocol (drafted originally for second and third trimester exams), and we tabulated the time required to do this. In the first 116 patients studied, we were able to clear every required portion of the fetal anatomy more than 90% of the time, with the exception of the kidneys (36%). The heart, however, requires special mention. With color Doppler we could obtain adequate four chamber views in 80% of the cases, crossing of the great vessels 54% of the time, and adequate three vessel views in 64% of cases. The average time required to accomplish the whole survey was 22 minutes, which included discussion of the findings along the way and performing the NT and nasal bone examinations according to Nuchal Translucency Quality Review guidelines.
Without data, it would be hard to argue that the first trimester scan should replace the second trimester anatomy survey, but it seems short sighted, even from a cost perspective, to limit ultrasound in a low-risk population to one per patient.
The take home messages from this meta-analysis are:
- More than half of fetal major anomalies can be identified between 11 and 14 weeks.
- The detection rates are even better if one suspects an anomaly — e.g., another anomaly has been noted, high-risk factors are present, or when a targeted examination, like an echocardiogram, is undertaken.
- A combined approach of TVS and TAS was somewhat more effective than either approach alone.