First-Trimester Ultrasound: A Comprehensive Guide

6. First-Trimester Ultrasound: Guidelines

Jude P. Crino  and Robert M. Ehsanipoor2

(1)

Department of Gynecology and Obstetrics, Johns Hopkins University School of Medicine, Phipps 228, 600 North Wolfe Street, Baltimore, MD 21287, USA

(2)

Department of Obstetrics and Gynecology, Sinai Hospital of Baltimore, Baltimore, MD, USA

Jude P. Crino

Email: jcrino1@jhmi.edu

Keywords

First trimesterGuidelinesObstetric ultrasoundPregnancyViabilityGestational ageMultiple gestationFetal anatomyNuchal translucencyDocumentationTraining

Guidelines and recommendations for the performance of first-trimester ultrasound have been developed and published by several societies and organizations, including the American College of Obstetricians and Gynecologists (ACOG), American College of Radiology (ACR), American Institute of Ultrasound in Medicine (AIUM), Australasian Society for Ultrasound in Medicine (ASUM), Hong Kong College of Obstetricians and Gynaecologists (HKCOG), International Society of Ultrasound in Obstetrics and Gynecology (ISUOG), National Institute of Child Health and Human Development (NICHD), Society for Maternal-Fetal Medicine (SMFM), Society of Obstetricians and Gynaecologists of Canada (SOGC), Society for Reproductive Endocrinology and Infertility (SREI), and Society of Radiologists in Ultrasound (SRU) [110]. These guidelines and recommendations have been published in a variety of printed and online formats. Several of these organizations have published collaborative guidelines. This chapter summarizes the key components of the various first-trimester ultrasound guidelines and recommendations. All concepts are not covered in each guideline, and pertinent differences between the published guidelines are described.

Equipment

Most guidelines state that studies should be conducted with real-time scanners, using a transabdominal or transvaginal approach. The ISUOG guidelines specify minimum capabilities of the equipment, including real-time, gray-scale, two-dimensional ultrasound, transabdominal and transvaginal ultrasound transducers, adjustable acoustic power output controls with output display standards, freeze frame and zoom capabilities, electronic calipers, capacity to print/store images, and regular maintenance and servicing [5]. Fetal exposure times should be minimized, using the shortest scan times and lowest possible power output needed to obtain diagnostic information. Doppler examination should only be used in the first trimester if clinically indicated [1112].

Indications

The most comprehensive list of indications for first-trimester ultrasound is published in the collaborative ACOG/ACR/AIUM/SRU guideline [1]. These indications are listed in Table 6.1. In asymptomatic women with a known last menstrual period (LMP), it remains controversial whether or not to offer routine first-trimester ultrasound to confirm an ongoing early pregnancy.

Table 6.1

Indications for first-trimester ultrasound examination [1]

Confirmation of the presence of an intrauterine pregnancy

Evaluation of a suspected ectopic pregnancy

Defining the cause of vaginal bleeding

Evaluation of pelvic pain

Estimation of gestational (menstrual) age

Diagnosis or evaluation of multiple gestations

Confirmation of cardiac activity

Imaging as an adjunct to chorionic villus sampling, embryo transfer, and localization and removal of an intrauterine device

Assessing for certain fetal anomalies, such as anencephaly, in high-risk patients

Evaluation of maternal pelvic masses and/or uterine abnormalities

Measuring the nuchal translucency (NT) when part of a screening program for fetal aneuploidy

Evaluation of a suspected hydatidiform mole

Timing of First-Trimester Ultrasound

The first trimester is defined as a gestational age of up to 13 6/7 menstrual weeks of gestation. The embryonic period encompasses the first 10 menstrual weeks, and some guidelines state that the term “embryo” should be used before 10 weeks and “fetus” thereafter. For some indications and purposes, the timing of the ultrasound requires more specific limits. For example, aneuploidy screening and evaluation of fetal gross anatomy should be performed between 11 and 13 6/7 weeks of gestation, and pregnancy dating is more accurate earlier in the first trimester [513].

Content of the Examination

Components of a standard first-trimester examination are assessment of pregnancy location, fetal number, fetal viability, measurements, determination of gestational age, and assessment of other intrauterine and extrauterine structures. Assessment of fetal anatomy and fetal aneuploidy assessment may be appropriate for some patients.

Pregnancy Location

The location of the pregnancy should be determined and documented. A definitive diagnosis of intrauterine pregnancy can be made when an intrauterine gestational sac containing a yolk sac or embryo/fetus with cardiac activity is visualized. Follow-up sonography and/or serial determination of maternal serum human chorionic gonadotropin levels are appropriate in pregnancies of undetermined location (see also Chap. 10) [1].

Fetal Number

Although the visualization of multiple sacs early in the first trimester is suspicious, the diagnosis of a multiple pregnancy requires visualization of multiple embryos/fetuses. The first trimester is the optimum time to determine chorionicity and amnionicity, which are critical for management of multi-fetal pregnancies. The presence of separate sacs, the thickness of the intervening membrane, and the shape of its junction with the placenta should be assessed [2] (Fig. 6.1). Early in the first trimester, an intervening amnion may not be visible in diamniotic–monochorionic twins [2]. Amnionicity and chorionicity should be stated in the ultrasound report.

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Fig. 6.1

(a) Monochorionic twin pregnancy at 9 weeks of gestation. Note the thin intervening membrane with no placental tissue. (b) Dichorionic twin pregnancy at 12 weeks of gestation. Note the thick intervening membrane with placental tissue (“twin peak” sign)

Assessment of Viability

Although the term “viability” implies the ability to live independently outside of the uterus, from an ultrasound perspective, the term is used to describe the presence of an embryo with cardiac activity at the time of the examination [5]. An alternative definition, proposed by the SRU, is a pregnancy that can potentially result in a liveborn baby. Updated diagnostic criteria for pregnancy viability have recently been published by the SRU due to occurrences of women diagnosed with miscarriage or ectopic pregnancy that resulted in damaging interventions to potentially normal pregnancies [10]. Findings diagnostic of pregnancy failure using transvaginal ultrasound include a crown-rump length (CRL) of ≥7 mm and no heartbeat, mean sac diameter (MSD) of ≥25 mm and no embryo, absence of an embryo with a heartbeat ≥2 weeks after a scan that showed a gestational sac without a yolk sac, and absence of an embryo with a heartbeat ≥11 days after a scan that showed a gestational sac with a yolk sac. Findings suspicious for, but not diagnostic of, pregnancy failure using transvaginal ultrasound include a CRL of <7 mm and no heartbeat, MSD of 16–24 mm and no embryo, absence of an embryo with a heartbeat 7–13 days after a scan that showed a gestational sac without a yolk sac, absence of an embryo with a heartbeat 7–10 days after a scan that showed a gestational sac with a yolk sac, absence of an embryo ≥6weeks after LMP, empty amnion (amnion seen adjacent to yolk sac, with no visible embryo), enlarged yolk sac (>7 mm), and a small gestational sac in relation to the size of the embryo (<5 mm difference between MSD and CRL). This group has also proposed that specific terminology be used, as summarized in Table 6.2.

Table 6.2

Terminology used early in the first trimester of pregnancya

Terminology

Comments

Viable

A pregnancy is viable if it can potentially result in a liveborn baby

Nonviable

A pregnancy is nonviable if it cannot possibly result in a liveborn baby. Ectopic pregnancies and failed intrauterine pregnancies are nonviable

Intrauterine pregnancy of uncertain viability

A woman is considered to have an intrauterine pregnancy of uncertain viability if transvaginal ultrasonography shows an intrauterine gestational sac with no embryonic heartbeat (and no findings of definite pregnancy failure)

Pregnancy of unknown location

A woman is considered to have a pregnancy of unknown location if she has a positive urine or serum pregnancy test and no intrauterine or ectopic pregnancy is seen on transvaginal ultrasonography

aReprinted with permission from Doubilet PM, Benson CB, Bourne T, Blaivas M. Diagnostic criteria for nonviable pregnancy early in the first trimester. Ultrasound Quarterly 2014 Jan; 30(1)

Early Pregnancy Measurements

The MSD and CRL should routinely be measured and reported. The MSD is the average of three orthogonal measurements of the fluid filled space within the gestational sac (Fig. 6.2). The CRL is the maximum length of the entire embryo or fetus measured as a straight line in a true midsagittal plane (Fig. 6.3). Later in the first trimester, care should be taken to ensure that the fetus is not flexed by looking for the presence of fluid between the fetal chin and chest (Fig. 6.4). The ISUOG guidelines state that the biparietal diameter (BPD) and head circumference (HC) may also be measured after 10 weeks of gestation [5].

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Fig. 6.2

Measurement of the mean gestational sac diameter in three orthogonal planes at 9 weeks of gestation

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Fig. 6.3

Measurement of the crown-rump length at 7 weeks of gestation

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Fig. 6.4

Measurement of the crown-rump length at 13 weeks of gestation. Note the fluid between the fetal chin and chest, confirming that the fetus is not flexed

Assessment of Gestational Age

By convention, the term “gestational age” refers to menstrual age and represents post-conception (post fertilization) age plus 14 days [5]. First-trimester measurement of the CRL is the most accurate method to establish or confirm gestational age, with an accuracy of ±5–7 days. Gestational age determination by CRL is more accurate than MSD; therefore, MSD should not be used to determine the expected due date. A recent collaborative ACOG/AIUM/SMFM committee opinion on the method for estimating the due date specifies guidelines for re-dating based on ultrasonography [13]. Discrepancies between ultrasound dating and LMP dating that support re-dating based on CRL measurement are more than 5 days at ≤8 6/7 weeks, and more than 7 days from 9 0/7 weeks to 13 6/7 weeks.

Assessment of Fetal Anatomy

Published guidelines differ significantly on the issue of fetal anatomy assessment in the first trimester. The collaborative ACOG/ACR/AIUM/SRU guideline states only that embryonic/fetal anatomy appropriate for the first trimester should be assessed [1]. The ISUOG guidelines, on the other hand, suggest a more specific and detailed anatomical assessment between 11 and 13 6/7 weeks, including the head, neck, face, spine, chest, heart, abdomen, abdominal wall, extremities, placenta, and umbilical cord, but state that such an assessment should only be performed if requested [5]. The ISUOG suggested anatomical assessment is summarized in Table 6.3.

Table 6.3

ISUOG suggested anatomical assessment at time of 11–13 + 6-week scana

Organ/anatomical area

Present and/or normal

Head

Present

Cranial bones

Midline falx

Choroid-plexus-filled ventricles

Neck

Normal appearance

Nuchal translucency thickness (if accepted after informed consent and trained/certified operator available)b

Face

Eyes with lensb

Nasal boneb

Normal profile/mandibleb

Intact lipsb

Spine

Vertebrae (longitudinal and axial)b

Intact overlying skinb

Chest

Symmetrical lung fields

No effusions or masses

Heart

Cardiac regular activity

Four symmetrical chambersb

Abdomen

Stomach present in left upper quadrant

Bladderb

Kidneysb

Abdominal wall

Normal cord insertion

No umbilical defects

Extremities

Four limbs each with three segments

Hands and feet with normal orientationb

Placenta

Size and texture

Cord

Three-vessel cordb

aReprinted from ISUOG Practice Guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol 2013;41(1):102–113, with permission from John Wiley & Sons

bOptional structures

Fetal Aneuploidy Assessment

The guidelines state that, when requested for patients desiring to assess their individual risk of fetal aneuploidy, measurement of the nuchal translucency (NT) combined with serum biochemistry may be performed. NT implementation requires several elements to be in place, including suitable equipment, counseling and management, as well as operators with specialized training and continuing certification [5]. First-trimester evaluation of the fetal nasal bone, screening for tricuspid regurgitation, and Doppler evaluation of the ductus venosus can also be used to screen for aneuploidy.

Other Intrauterine and Extrauterine Structures

There is a general consensus that the uterus, cervix, adnexa, and cul-de-sac region should be examined, and pathologic findings such as abnormalities of uterine shape, fibroids, and adnexal masses should be imaged and documented. The ISUOG guidelines further recommend that the area between the bladder and the uterine isthmus be scrutinized in women with a prior cesarean section. They also recommend that the echo-structure of the placenta should be evaluated, and that placenta previa should not be reported at this stage [5].

Documentation

The AIUM offers specific guidelines regarding documentation of ultrasound examinations [14]. There should be a permanent record of the ultrasound examination and its interpretation. Appropriately labeled relevant images should be recorded in a retrievable format. A signed final report of the ultrasound findings is included in the patient’s medical record and is the definitive documentation of the study. Final reports should be available within 24 h of completion of the examination or, for non-emergency cases, by the next business day. Specific documentation requirements will vary depending on the type of exam, indication, findings, and whether or not any associated procedures were performed. If the results of the examination are important and unexpected, or require urgent intervention, communication should occur directly between the interpreting physician and the patient’s health care provider.

Training Guidelines

The ISUOG has published recommendations for basic training in obstetric and gynecologic ultrasound [15]. It states that formal basic teaching should include three steps: theoretical training, practical training, and examination. Theoretical training is primarily didactic and should cover physics, basics of diagnostic ultrasound, and pertinent clinical concepts. This is followed by practical training with formal supervision, which should include a method of confirming that ultrasounds were performed and documented in a standardized way. The examination should assess theoretical knowledge and may be complemented by a practical examination. The AIUM has published training guidelines for physicians who evaluate and interpret diagnostic obstetric ultrasound examinations [16]. A physician should be involved with the performance, evaluation, interpretation, and reporting of a minimum of 300 diagnostic obstetric ultrasound examinations during completion of an approved training program. For physicians who did not receive ultrasound training during residency and/or fellowship, a minimum of 50 AMA PRA Category I Credits dedicated to diagnostic obstetric ultrasound and involvement with at least 300 exams under the supervision of a qualified physician within a 36 month period is recommended.

Cleaning and Preparing Transducers

The AIUM has published guidelines for cleaning and preparing ultrasound transducers between patients, including specific procedures for cleaning and disinfection [17]. External probes that only come into contact with clean, intact skin require cleaning after each use. Internal probes should be covered for each exam. Variable rates of leakage have been reported with both condoms and commercially available covers and therefore high-level disinfection of the probe is required after each use. Interestingly, condoms have lower leakage compared to commercially available probe covers and also have better acceptable quality levels.

Teaching Points

·               Guidelines and recommendations for the performance of first-trimester ultrasound have been developed and published by several societies and organizations.

·               Components of a standard first-trimester examination are assessment of pregnancy location, fetal number, fetal viability, measurements, determination of gestational age, and assessment of other intrauterine and extrauterine structures. Assessment of fetal anatomy and fetal aneuploidy assessment may be appropriate for some patients.

·               The first trimester is the optimum time to determine chorionicity and amnionicity, which are critical for management of multi-fetal pregnancies.

·               Findings diagnostic of pregnancy failure include a crown-rump length (CRL) of ≥7 mm and no heartbeat, mean sac diameter (MSD) of ≥25 mm and no embryo, absence of an embryo with a heartbeat ≥2 weeks after a scan that showed a gestational sac without a yolk sac, and absence of an embryo with a heartbeat ≥11 days after a scan that showed a gestational sac with a yolk sac.

·               Discrepancies between ultrasound dating and LMP dating that support re-dating based on CRL measurement are more than 5 days at ≤8 6/7 weeks, and more than 7 days from 9 0/7 weeks to 13 6/7 weeks.

References

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2.

Australasian Society for Ultrasound in Medicine. Guidelines for the performance of first trimester ultrasound. 1995. http://www.asum.com/au/newsite/Resources.php?p=Policy. Accessed Aug 2012.

3.

The Hong Kong College of Obstetricians and Gynaecologists. Guidelines for first trimester ultrasound examination: part I. 2004. http://www.hkcog.org.hk/hkcog/pages_4_81.html. Accessed Mar 2004.

4.

The Hong Kong College of Obstetricians and Gynaecologists. Guidelines for first trimester ultrasound examination: part II. 2004. http://www.hkcog.org.hk/hkcog/pages_4_81.html. Accessed Mar 2004.

5.

ISUOG. Practice guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2013;41(1):102–13.CrossRef

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Reddy UM, Abuhamad AZ, Levine D, Saade GR. Fetal Imaging Workshop Invited Participants. Fetal imaging: executive summary of a joint Eunice Kennedy Shriver National Institute of Child Health and Human Development, Society for Maternal-Fetal Medicine, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound Fetal Imaging workshop. J Ultrasound Med. 2014;33(5):745–57.CrossRefPubMed

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Demianczuk NN, Van Den Hof MC, Farquharson D, Lewthwaite B, Gagnon R, Morin L, et al. The use of first trimester ultrasound. J Obstet Gynaecol Can. 2003;25(10):864–75.PubMed

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Morin L, Van den Hof MC, Diagnostic Imaging Committee, Society of Obstetricians and Gynaecologists of Canada. Ultrasound evaluation of first trimester pregnancy complications. J Obstet Gynaecol Can. 2005;27(6):581–91.PubMed

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American Institute of Ultrasound in Medicine, Society for Reproductive Endocrinology and Infertility, American Society of Reproductive Medicine. AIUM practice guideline for ultrasonography in reproductive medicine. J Ultrasound Med. 2009;28(1):128–37.

10.

Doubilet PM, Benson CB, Bourne T, Blaivas M. Diagnostic criteria for nonviable pregnancy early in the first trimester. N Engl J Med. 2013;369(15):1443–51.CrossRefPubMed

11.

American Institute of Ultrasound in Medicine [Internet]. Laurel, MD: Official statement; statement on the safe use of doppler ultrasound during 11–14 weeks scans (or earlier in pregnancy). http://www.aium.org/officialStatements/42.

12.

ISUOG. Practice guidelines: use of Doppler ultrasonography in obstetrics. Ultrasound Obstet Gynecol. 2013;41(2):233–9.CrossRef

13.

ACOG. Committee opinion no 611: method for estimating due date. Obstet Gynecol. 2014;124(4):863–6.CrossRef

14.

American Institute of Ultrasound in Medicine (AIUM). AIUM practice guideline for documentation of an ultrasound examination. J Ultrasound Med. 2014;33(12):2219–24.CrossRef

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ISUOG. Education Committee recommendations for basic training in obstetric and gynecological ultrasound. Ultrasound Obstet Gynecol. 2014;43(1):113–6.CrossRef

16.

American Institute of Ultrasound in Medicine [Internet]. Laurel, MD: Official statement; training guidelines for physicians who evaluate and interpret diagnostic obstetric ultrasound examinations. http://www.aium.org/resources/viewStatement.aspx?id=59.

17.

American Institute of Ultrasound in Medicine [Internet]. Laurel, MD: Official statement; guidelines for cleaning and preparing external- and internal-use ultrasound probes between patients. http://www.aium.org/officialStatements/57.