In the late 1980s and early 1990s, ultrasound evaluation of the fetus at less than 16 weeks of gestation was made possible by the advent of high-resolution transvaginal transducers.¹, ², ³, ⁴, ⁵, ⁶, ⁷ With the introduction of transvaginal ultrasound, several reports evaluated the feasibility of this approach in the first trimester and demonstrated the ability to assess normal and abnormal anatomy of the fetal brain, heart, kidneys, and other organs.¹, ², ³, ⁴, ⁵, ⁶, ⁷ The observation of the relationship between the presence of increased fluid in the fetal neck region in the first trimester and chromosomal abnormalities resulted in the establishment of nuchal translucency (NT) as an ultrasound screening tool for aneuploidy.⁸, ⁹, ¹⁰ Largely through the efforts of Dr. Nicolaides and his coworkers, the NT measurement was standardized and a first trimester screening strategy program was established.¹⁰, ¹¹, ¹², ¹³ Consistency and reliability of NT was ensured through standardization of measurement and with the establishment of quality assurance programs.¹⁴,¹⁵ Over the past two decades, the first trimester NT ultrasound examination has evolved beyond aneuploidy screening and now includes an evaluation of fetal anatomy in early gestation. Recently published guidelines reflect this development.¹⁶,¹⁷ Familiarization with existing standardization of measurements and with national/international guidelines is an important step in the performance of the first trimester ultrasound examination. Given that knowledge in this field is evolving at a rapid pace, we recommend that ultrasound practitioners stay abreast of the literature on this subject. In this chapter, we present information on standardization of ultrasound measurements in the first trimester and report on existing guidelines. It is important to note that with new evidence, guidelines change over time and the readers are encouraged to refer to the most current version as reference.

It is important to understand the various terms that are used in standardization of ultrasound practice. Guidelines, protocols, standards, and policies refer to the ultrasound examination itself (the NT screening or the first trimester anatomy survey). Certifications, credentialing, and qualifications refer to the personnel performing the ultrasound examinations including physicians, sonographers, and allied health personnel. Accreditation, on the other hand, refers to the ultrasound laboratory/unit where the examination is performed and thus requires evaluation of the qualifications of the personnel performing the ultrasound examination, the equipment that is being used for the ultrasound examination, compliance with existing examination guidelines, and quality assurance.

The last 20 years have shown that standardizing the approach to NT, nasal bone, tricuspid regurgitation, and ductus venosus in the first trimester has increased the reliability and reproducibility of these measurements.¹³ Recently published guidelines on first trimester ultrasound incorporate the NT and emphasize the role that the first trimester ultrasound plays in the assessment of fetal anatomy.¹⁶,¹⁷ In general, guidelines are consensus based and reflect on the scientific evidence at the time of guideline development. Guidelines reduce inappropriate variations in practice and provide a more rational basis for study referral. Guidelines also, when appropriately developed, provide a focus for quality control and a need for continuing medical education for the personnel performing the ultrasound examination. Guidelines may also identify shortcomings of scientific studies and suggest appropriate research topics on the subject.