Ultrasound in Obstetrics & Gynecology | 2021
Integration of nuchal translucency screening into the first‐trimester fetal anatomy scan: the time has come
Abstract
The last 30 years have seen the parallel development of two key components of the armamentarium of prenatal diagnosis: fetal anatomy scanning (FAS) and nuchal translucency (NT) screening. These two tools were developed with different goals in mind: NT was introduced originally as a screening test for the risk of trisomy 21, and FAS as a means to assess the anatomy of the embryo and fetus. FAS preceded NT onto the scene, as it did midtrimester maternal biochemical screening. With the ongoing improvement in ultrasound equipment and the accrual of knowledge on the sonographic appearance of normal and anomalous anatomy, the timing of FAS shifted earlier in pregnancy. The clinical application of transvaginal ultrasound (TVS) in obstetrics and gynecology has been around since the 80s. The introduction of high-frequency transducers mounted on practicable transvaginal probes provided a close-up look at the uterus, adnexa, cervix and developing embryo. Its usefulness was quickly recognized in gynecology, and it soon proved its worth in clinical practice, for example in in-vitro fertilization for scanning the pelvic organs, tracking follicle maturation and guiding invasive procedures. Its adoption in prenatal diagnosis, however, encountered a certain amount of resistance from both patients and practitioners. This began to change with the groundbreaking studies of Timor-Tritsch et al.1,2 and Rottem and Bronshtein and colleagues3–5, which demonstrated the diagnostic feasibility of the transvaginal approach to produce clinically actionable images of the embryo. Bronshtein et al.4 scanned patients at 9–15 weeks’ gestation, distinguished cystic nuchal hygroma of different types and demonstrated the association between this collection of nuchal fluid and fetal trisomy 21. These studies opened the door to a new era of early prenatal screening. The seminal study by Nicolaides and colleagues6 showed that a third of fetuses with NT > 3 mm and ≤ 8 mm were subsequently diagnosed with aneuploidy. Within a short time, screening programs for fetal aneuploidy based on NT were established7. It was soon noted that other defects and syndromes, particularly congenital heart defects, occurred in greater frequency in the chromosomally normal fetuses which had increased NT8. These findings were combined with other maternal and fetal parameters to create first-trimester screening programs for the risk of aneuploidy and other anomalies. Concomitant with the expansion of these screening programs, the applications and scanning capabilities of first-trimester ultrasound, mostly TVS, continued to grow. Machines and transducers improved and more centers honed their expertise in fetal imaging at these early gestational ages. Rottem and Bronshtein and other researchers published large cohort studies of transvaginal, early, targeted organ scanning performed between 9 and 16 weeks5. We and others showed that transvaginal scanning of the fetal heart could be performed reliably in the late first and early second trimesters, applying the same extended protocols used in midtrimester examinations9–13. Inevitably, attempts were made to marry first-trimester screening with FAS, by moving FAS to the ‘NT window’. However, even in the hands of leading practitioners, FAS at 11 to 13 + 6 weeks presented unique challenges8. Detection rates (DR) for fetal structural anomalies were low overall. In addition, it soon became evident that an array of developmental malformations were not amenable to early visualization, since they were not fully realized at these gestational ages. Certain lesions may worsen progressively over a period of weeks as their impact gradually becomes apparent (e.g. aortic stenosis)9,10,14, while normal anatomy might not yet be available for assessment (e.g. the corpus callosum). It was clear that early fetal scanning, no matter how much it improved, would never replace the midtrimester scan. In fact, a third-trimester scan can similarly detect anomalies that are not amenable to diagnosis at the midtrimester scan10,14,15. Another barrier to the success of FAS during the NT window was the reluctance of many practitioners, whether from lack of experience or confidence, or in response to perceived patient preference, to perform TVS, preferring to rely on the traditional transabdominal route. Yet, despite these difficulties and setbacks, FAS continued to be offered at increasingly early gestational ages and, in 2013, ISUOG published a guideline for the performance of early FAS16, including ‘required’, ‘recommended’ and ‘optional’ scanning targets. Today, early FAS seems to be coming of age. Syngelaki and colleagues17 recently published a prodigious study encompassing over 100 000 participants who underwent FAS during the NT window. Transabdominal transducers have improved since the original attempts at early scanning; these authors reported that transvaginal scanning was necessary to complete the standardized exam protocol in only 2–3% of cases. They stressed