Chapter 5: Ultrasound of the Fetus at 11 to 18 Weeks

INTRODUCTION

The anatomical survey, as well as the understanding of the developmental anatomy and detection rates of fetal anomalies, has evolved over the last 30 years as the result of more advanced and sophisticated ultrasound technologies. Over the last 25 years, 18 weeks has been considered by many to be the ideal or “gold-standard” gestational age to perform the first, and at times the only, anatomical survey. There are numerous advantages to performing the scan at this gestational age: the fetus is large enough to be imaged easily using transabdominal (TA) sonography, and most anomalies are present and can be detected. The disadvantage of using this gestational age for the anatomical survey is that maternal body characteristics such as abdominal obesity, uterine fibroids, and fetal position can at times preclude adequate imaging of the fetal structures, which can result in an incomplete scan. This, in turn, necessitates a follow-up scan. This need for follow-up scans has resulted in a gradual shift of the 18-week anatomy scan to be performed at 20 to 22 weeks. Therefore, the “18-week anatomy scan” has become a brand name for the sometimes only detailed anatomical survey that in many practices is carried out typically at around 20 weeks or anytime between 18 and 22 weeks of gestation.

Many of the fetal anomalies seen at the 18-week anatomy scan are present starting at the first and/or early second trimesters and, if carefully looked for, can be detected. In the 1990s, the early anatomy scan was performed mostly using transvaginal sonography (TVS) between 14 and 17 weeks of gestation. Although during this scan the normal early fetal anatomy can be seen and anomalies can be detected, it never reached popularity, and few practices adopted it as a routine¹. The “11- to 13-week scan” also known as the “nuchal scan” is part of the first trimester screening for Down syndrome and continues to have increasing acceptance among patients and practitioners even in the era of noninvasive prenatal screening (NIPS) using cell-free fetal DNA from maternal blood. In 1997, D’Ottavio et al² in a study a total of 3514 fetuses reported a detection rate of a single scan at 13-15 weeks to be 66.7%, and in combination with a 20-22 weeks scan its 90.5%. In another study, this time a prospective observational study, which aimed to determine the efficacy of the first-trimester anomaly scan, Becker and Wegner³ scanned 3094 consecutive fetuses between 11 and 13 weeks and 6 days with an 83.7% detection rate of major anomalies. Ebrashy et al⁴ in prospective study of 2876 patients found that all anomalies that were detected at the first- and/or midtrimester scans were confirmed and documented after delivery. At the 13- to 14-week scan they diagnosed 21 (68%) of the 31 cases diagnosed with anomalies prenatally. Recently, Bromley et al⁵ reported a 41.4% detection rate of malformations at 11 to 13 weeks and 6 days ...