ULTRASOUND EVALUATION DURING THE FIRST TRIMESTER OF PREGNANCY

6 ULTRASOUND EVALUATION DURING THE FIRST TRIMESTER OF PREGNANCY




During the first trimester of pregnancy, a unique and dramatic sequence of events occurs, defining the most critical and tenuous period of human development: the remarkable transformation of a single cell into a recognizable human being. The time span for the first trimester is based on menstrual dates; in a patient with a 28-day cycle, it begins 2 weeks before fertilization (on the first day of the last normal menstrual period) and concludes 12 weeks later. In this chapter, gestational age (GA) is used exclusively and is synonymous with menstrual age (MA). This terminology is chosen because menstruation is a visible event that establishes the date of the last normal menstrual period, and because it is consistent with radiologic and obstetric usage. In contrast, embryologic dating begins with conception and is, therefore, 2 weeks later than the first day of the last normal menstrual period. Although embryologic age is scientifically more precise, there is no visible landmark to announce conception, and so, radiologists and obstetricians continue to use MA or GA for pregnancy dating.


Transvaginal sonography is the optimal way to image a patient during the first trimester of pregnancy (Fig. 6-1). Indications and the need to do this examination are many and include1: (1) to identify the location and number of gestational sacs; (2) to assign a GA to the pregnancy; (3) to determine whether an early pregnancy has a normal appearance or whether sonographic indicators are present that predict failure; (4) to evaluate maternal symptoms such as pain or bleeding; (5) to evaluate uterine contents before pregnancy termination; and (6) to guide diagnostic or therapeutic procedures that require visual guidance (that is, chorionic villus sampling, amniocentesis). As ultrasound technology continues to evolve and improve, there is increasing emphasis for early screening of fetal complications. For example, early detection of Down syndrome and other chromosomal abnormalities can now be achieved by measuring the fetal nuchal translucency between 11 and 14 weeks’ gestation, and combining this information with maternal age and maternal serum biochemistry levels (pregnancy-associated plasma protein A [PAPP-A], and the free beta subunit of human chorionic gonadotropin).2,3 Some investigators also suggest that it is possible to examine cardiac and noncardiac fetal anatomy in a low-risk population in the setting of a routine 11 to 14 week ultrasound scan.4 Indeed, at this GA, a wide range of congenital anomalies that can affect the central nervous system, heart, ventral wall, urinary tract, and skeleton have been reported.5 What role three-dimensional (3D) ultrasound will play at this time of development is still unclear, but preliminary work suggests that it can potentially minimize scanning time and provide an excellent way to store scanned data.6



To comprehend normal and abnormal sonographic findings in early pregnancy, it is important to understand normal development and to appreciate the rapid and critical sequential changes that are occurring. The first trimester can be divided into preovulation and periovulation, conceptus, embryonic, and fetal phases of development (Table 6-1).7 The following discussion highlights physiologic, embryologic, and anatomic changes that occur during this time and emphasizes developmental changes as they relate to sonographic images obtained with high-resolution transvaginal transducers. A 28-day cycle is used in this discussion, although a normal cycle may vary in length from 25 to 35 days. For those interested in a more detailed explanation of these events, an excellent source for this material is The Developing Human: Clinically Oriented Embryology by Moore and Persaud.818


Table 6-1 Embryology of Early Pregnancy









































































Period of Development Weeks Features
Pre- and periovulation 1–2 Ovarian follicle matures
Ovulation
   
Corpus luteum
Conceptus 3–5 Fertilization
   
Morula
   
Blastocyst
   
    Trilaminar embryo (flat embryo)
Embryonic 6–10 C-shape embryo
   
Major organs develop
   
Yolk sac detaches
Fetal 11–12 Fetal growth
   
    Amniotic and chorionic membranes approach each other (fusion at 17 weeks)

Adapted from Sohaey R, Woodward P, Zweibel WJ: First-trimester ultrasound: The essentials. Semin Ultrasound CT MR 17:2, 1996.



MATERNAL PHYSIOLOGY AND EMBRYO DEVELOPMENT


During the first 2 weeks of pre- and periovulation, cyclic changes occur within both the ovaries and endometrium as a result of the influence of pituitary gonadotropic follicle-stimulating hormone (FSH) and luteinizing hormone (LH) (Fig. 6-2).17 Initially, under the influence of FSH, a mature ovarian follicle develops. Estrogen elaborated by the follicle causes the functional layer of endometrium to proliferate and become thicker, as the spiral arteries elongate and the uterine glands increase in number and length. As a result of an abrupt surge in LH, ovulation occurs, and an oocyte is extruded, typically on day 14 of the cycle. After ovulation, the follicle collapses and transforms into the glandular corpus luteum, which produces progesterone and a small amount of estrogen. This hormonal activity is responsible for additional histologic changes of the endometrium as it enters the secretory phase, so named because the uterine glands now secrete material rich in glycogen. The glands become increasingly wide, tortuous, and saccular, and the uterine spiral arteries become increasingly coiled as they invade the superficial compact layer of endometrium. The endometrium continues to thicken as a result of glandular and vascular growth and increased stromal fluid.



During the 3rd to 5th weeks of the cycle (the conceptus period), fertilization occurs, with subsequent development of the morula, blastocyst, and bilaminar and ultimately trilaminar, or flat, embryo.7,8,10,16 Fertilization most often occurs within 1 day of ovulation (day 15 of the 28-day cycle), typically in the ampulla, the longest and widest portion of the fallopian tube (Fig. 6-3). At fertilization, fusion of the egg and sperm, each haploid gamete with 23 individual chromosomes, results in a zygote, a diploid cell with 23 pairs, or 46 chromosomes. Over the next 2 days, the cell mass transgresses the tube while dividing repeatedly to form a solid ball of 12 or more cells, called the morula. As the morula enters the uterine cavity on day 18 or 19 of the cycle, endometrial fluid penetrates the cell mass to create a central cavity. When this occurs, the morula is transformed into a blastocyst, and its tissue is divided into two important layers. The outer cell layer, or trophoblast, ultimately creates the chorionic membranes and the fetal contribution to the placenta. The inner cell layer develops into the embryo, amnion, umbilical cord, and the primary and secondary yolk sacs. By the end of the 3rd week, the blastocyst begins to implant into the decidualized endometrium, a term applied to the functional layer of the thickened and edematous gravid endometrium (Fig. 6-4).




During the 4th week, the blastocyst, measuring only 1 mm in diameter, becomes fully imbedded into endometrial tissue. Not surprisingly, during this process, as trophoblastic tissue invades the endometrium, vaginal bleeding may occur and be confused clinically with an atypical menstrual cycle. The 4th week is a time of rapid cell proliferation and differentiation, affecting multiple primordial structures. The primary yolk sac shrinks and disappears gradually while the secondary yolk sac forms. The latter structure plays a critical role by providing nutrients for the embryo, serving as the site for initial hematopoiesis, and contributing to the developing gut and reproductive systems.9 A tiny bilaminar embryo also forms between the secondary yolk sac and developing amnion, and a primitive ureteroplacental circulation is established.8 By the end of this week, the products of conception have attained a diameter of 2 to 3 mm and are at the threshold of detection by state-of-the-art transvaginal ultrasound transducers. In addition, the pregnancy test becomes positive because a measurable quantity of human chorionic gonadotropin (hCG) is produced by trophoblastic tissue.


During the final week of the conceptus stage (5 weeks’ GA), normal menstrual flow is absent, and the woman may suspect pregnancy. The products of conception continue to enlarge primarily as a result of expansion of the chorionic cavity, which attains a diameter of 5 mm (Fig. 6-5). This cavity is identified by sonologists as fluid within the “gestational sac.” The secondary yolk sac is variably identified by sonographic examination, and the developing bilaminar embryonic disk undergoes the process of gastrulation, which transforms it into a trilaminar disk with three germ layers (endoderm, mesoderm, and ectoderm). Despite these transformations, the embryo remains undetectable by sonography.



Weeks 6 through 10 constitute the embryonic phase, during which time all major internal and external structures begin to form (Table 6-2).11 Although most organ function remains minimal, the cardiovascular system develops rapidly, and the primordial heart starts to beat at the beginning of the 6th week.12 The appearance of the embryo changes dramatically as it is transformed from its flat disk-like configuration to a C-shaped structure, and it develops a human-like appearance (Fig. 6-6). During embryogenesis, crown rump length (CRL) grows rapidly, measuring 30 mm by the end of the 10th week.


Table 6-2 Criteria for Estimating Developmental Stages in Human Embryos



































































GA (days) Length (mm) Main External Characteristics
34–35 1.5–3.0 Flat embryonic disk.
36–37 2.0–3.5 Embryo straight or slightly curved.
38–39 2.5–4.5 Embryo curved owing to head and tail folds.
40–41 3.0–5.0 Upper limb buds appear.
42–44 4.0–6.0 Embryo has C-shaped curve.
45–46 5.0–7.0 Upper limbs are paddle shaped.
47–50 7.0–9.0 Hand plates formed; digital rays present. Lower limbs are paddle shaped.
51–54 8.0–11.0 Foot plates formed.
55–57 11.0–14.0 Digital rays visible in hand plates.
58–60 13.0–17.0 Digital rays clearly visible in foot plates.
61–62 16.0–18.0 Limbs extend ventrally. Trunk elongating and straightening. Midgut herniation prominent.
63–65 18.0–22.0 Upper limbs longer and bent at elbows. Fingers webbed.
66–67 22.0–28.0 Hands and feet approach each other. Fingers are free and longer. Toes webbed. Stubbly tail present.
68–69 23.0–28.0 Toes free and longer. Eyelids and external ears more developed.
70 27.0–31.0 Head more rounded and shows human characteristics. External genitalia still have sexless appearance. Distinct bulge still present in umbilical cord, caused by herniation of intestines. Tail has disappeared.

GA, gestational age.


Adapted from Moore KL, Persaud TVN: Organogenic period: The fourth to eighth weeks. In Moore KL, Persaud TVN (eds): The Developing Human: Clinically Oriented Embryology, 7th ed. Philadelphia, WB Saunders, 2003, pp 78-93.



The final 2 weeks of the first trimester (11th and 12th weeks GA) begin the fetal period, during which there is continued rapid growth and ongoing organ development.13 During the initial phase of fetal development, the head is disproportionately large and constitutes one half of the CRL. As body growth subsequently accelerates, relative proportionality becomes apparent.



DEVELOPMENT OF THE PLACENTA AND FETAL MEMBRANES



Development of the Placenta


The placenta contains both maternal and fetal tissue. The maternal component is derived from a portion of decidualized endometrium, whereas the fetal component is derived from a portion of chorionic tissue that surrounds the blastocyst.14


Endometrium deep to the implanted conceptus, the decidua basalis, forms the maternal component of the placenta. The superficial portion of decidua, which covers the invading conceptus, is the decidua capsularis; the remaining endometrium is the decidua parietalis (also called decidua vera) (Fig. 6-7).



The fetal component of the placenta is derived from trophoblastic tissue, which by 5 weeks’ GA develops into chorionic villi that completely encircle the conceptus (see Fig. 6-7A).15 Initially, these villi are uniform in thickness and are in intimate contact with adjacent decidual tissue. Subsequently, the most superficial two thirds of villi, located immediately beneath the decidua capsularis, degenerate to form the smooth chorion (also called chorion laeve) (see Fig. 6-7B). The remaining villi are in contact with the decidua basalis and are located adjacent to the most deeply imbedded portion of the blastocyst. These villi rapidly increase in number, branch profusely, and enlarge as they become the chorion frondosum, the fetal contribution to the definitive placenta.



Development of the Ureteroplacental Circulation


To understand developing ureteroplacental circulation, a basic knowledge of the anatomy and physiology of uterine blood flow is helpful. Blood to the uterus is from the paired uterine arteries, which are branches of the anterior division of the internal iliac artery. As each uterine artery enters the uterus at the uterocervical junction, it ascends along the lateral uterine wall and produces multiple penetrating arcuate branches. When these branches pierce the endometrium, they become the spiral arteries.


With early embryonic development, the spiral arteries located within the decidua basalis become increasingly prominent. Side-by-side maternal and embryonic circulations are established initially as trophoblastic cells form chorionic villi that invade into portions of the decidualized endometrium (Fig. 6-8).14 Before actual maternal-fetal circulation is established, however, the invading trophoblastic cells create plugs within the maternal spiral arteries. The villi simultaneously erode tiny portions of the decidua, which subsequently enlarge to form the intervillous spaces. The intervillous spaces will ultimately receive maternal blood from the spiral arteries, and thus the villi containing fetal blood will be surrounded and perfused by maternal blood by the end of the first trimester.19 There is some controversy as to how and when in the first trimester the perfusion of villi begins and when actual maternal fetal circulation is initiated. Two hypotheses have been proposed to explain these events.



The first hypothesis is based on studies of first trimester gravid hysterectomy specimens and postulates that until the end of the first trimester there is no communication between maternal and fetal circulations, and intervillous blood flow is absent.2028 Early studies suggested that the intervillous spaces are initially filled with clear fluid, possibly filtered maternal plasma.20 These fluid-filled spaces may create a hypoxic and protective environment for the developing embryo.29 As the trophoblastic plugs within the spiral arteries dissolve or dislodge by the end of the first trimester, maternal blood begins to circulate into the intervillous spaces. Concurrent with this event is a marked increase in uterine blood flow, which can be documented by sonography.26


An alternative hypothesis suggests that transformation of the intervillous space from fluid filled to blood filled occurs gradually throughout the entire first trimester.30,31 According to this theory, the trophoblastic plugs are incomplete and venous flow is normally present in the intervillous space. The flow is low velocity and has low pulsatility, presumably to prevent the implanting trophoblast from becoming detached from the decidua. This slow flow can be detected by sensitive Doppler techniques by as early as 5.5 gestational weeks.3236 Which of these theories best approximates actual physiologic development remains to be determined.



Development of the Chorionic “Membrane”


The chorion is derived from the superficial two thirds of chorionic villi that, at approximately 10 weeks’ GA, become compressed and avascular, and subsequently degenerate into a membrane-like structure (see Fig. 6-7B).14 Embryologists identify this as the smooth chorion or chorion laeve, whereas ultrasonologists refer to it as the chorionic “membrane.” Because the chorionic membrane and chorion frondosum each originate from chorionic villi, these two structures remain in intimate contact throughout gestation. For this reason, the chorionic membrane always extends up to and merges with the edge of the placenta. As is described later, understanding this anatomic relationship is important for comprehending sonographic findings that accompany placental bleeding.


As the gestational sac grows, it begins to protrude into the compressed uterine cavity (see Fig. 6-7). As a result, the overlying decidua capsularis and smooth chorion come into contact with the decidua parietalis, which is located on the opposite side of the uterine cavity. During the second trimester, the capsularis and parietalis actually fuse and obliterate the uterine cavity. This allows the chorionic membrane to become loosely attached to the decidua parietalis. These anatomic relationships have clinical import because, in the event of placental bleeding, blood can easily dissect into the space between the chorionic membrane and decidua parietalis (see later discussion).14



Development of the Amnion


At 3 to 4 weeks’ GA, concurrent with implantation and development of trophoblastic tissue, the amnionic membrane begins to form from cells that originate from the inner blastocyst (see Figs. 6-4, 6-5, and 6-9).8 This membrane initially surrounds the newly formed amniotic cavity, opposite the newly formed secondary yolk sac, and is attached to the bilaminar embryonic disk, which is contiguous to and lies between the amnion and chorion. As the amnion and its cavity rapidly expand, they surround the growing embryo. The amniotic membrane remains attached to the embryo at the umbilical cord insertion site; as the embryo flexes, its dorsal surface pushes into the amniotic sac. As the umbilical cord elongates, the secondary yolk sac, having completed its functions, is displaced away from the embryo and is readily visible within the shrinking chorionic cavity. Whereas the amnionic cavity continues to expand, the chorionic cavity shrinks and is ultimately obliterated between 12 and 16 weeks.




NORMAL SONOGRAPHIC ANATOMY AND LANDMARKS


Despite considering the pre- and periovulation period as part of the “first trimester of pregnancy,” pregnancy has not yet occurred. However, each month the functional layer of endometrium undergoes anticipatory changes in the event of conception. During menstruation, sonographic images depict the endometrium as a thin central echogenic line because of the opposed endometrial surfaces. Over the next 10 days, before ovulation, the functional layer proliferates, and at sonography a hypoechoic area is perceived surrounding the central linear echo. The total anteroposterior thickness of this multilayered endometrium is approximately 8 mm. Correlation with histologic specimens suggests that this appearance is due to relative homogeneity of endometrial tissue, with the glandular elements remaining straight and orderly.37,38


During the conceptus phase of pregnancy (weeks 3 to 5), despite rapid and dramatic changes involving the products of conception, the endometrial appearance is identical to that observed during a nonconceptual cycle. After ovulation, it enters the secretory phase of the cycle, becoming dramatically hyperechogenic, and approaching 14 mm in total thickness. Histologically, the increased echogenicity results not only from new acoustic interfaces that originate as a result of glandular and vascular tortuosity but also from reflections from glandular secretions, glycogen, and mucus.37 As the 4th week concludes, despite complete implantation, the blastocyst remains undetectable to even high-resolution vaginal imaging because of its small size (1 mm). During the final week of the conceptus phase, however, just as the patient becomes aware that she may be pregnant, sonography can often identify changes that signal intrauterine implantation.



Identifying the Gestational Sac


The first definitive sonographic finding to suggest early pregnancy is visualization of the gestational sac. Using vaginal transducers with frequencies of at least 5 MHz, the size threshold for sac detection is 2 to 3 mm, corresponding to between 4 weeks’ and 1 day GA and 4 weeks’ and 3 days GA (Fig. 6-10).3941 At sonography, the earliest appearance of a gestational sac is a small round fluid collection surrounded completely by a hyperechogenic rim of tissue. The central fluid collection is the chorionic cavity, and the surrounding echoes are due to developing chorionic villi and adjacent decidual tissue. As the sac enlarges, the hyperechogenic rim should be at least 2 mm thick, and its echogenicity should exceed the level of myometrial echoes.42



To maintain uniformity, gestational sac size should be determined by calculating the mean sac diameter (MSD). This value is obtained by adding the three orthogonal dimensions of the chorionic cavity (excluding the surrounding echogenic rim of tissue) and dividing by 3 (Fig. 6-11).



The position of a normal gestational sac is in the mid- to upper uterus. As the sac implants into the decidualized endometrium, it should be adjacent to the linear central cavity echo complex, without initially displacing or deforming this hyperechogenic anatomic landmark (Fig. 6-12). On the basis of the physiology of sac implantation, Yeh et al,43 in 1986, described the intradecidual sign. Using this sign with a transabdominal approach, these investigators reported a sensitivity of 92%, specificity of 100%, and accuracy of 93% for diagnosing an early intrauterine pregnancy (IUP). Two subsequent studies, each using a vaginal approach, have been published in an attempt to validate the effectiveness of this sign. The initial report by Laing et al44 had disappointing results, with a sensitivity of 34% to 66%, specificity of 55% to 73%, and accuracy of 38% to 65%. A subsequent investigation by Chiang et al,45 had more favorable results, with a sensitivity of 60% to 68%, specificity of 97% to 100%, and accuracy of 67% to 73%. The reason for the disparate results of these two reports is uncertain, but may relate to different criteria used to precisely and confidently identify the thin echogenic line (central uterine stripe) that represents the potential uterine cavity.



Also confounding the problem of identifying an early intrauterine gestational sac with certainty, is that with continued growth, it often changes shape from round to elliptical, and it may develop an irregular contour as a result of adjacent uterine contractions, myomas, implantation bleeds, or a distended maternal urinary bladder. Because even the most favorable reported statistics are not highly sensitive or accurate for documenting the intradecidual sac sign, and because occasionally a pseudogestational sac of an ectopic pregnancy and a gestational sac can have a very similar appearance (Fig. 6-13), the value of this sign appears limited. In cases of very early pregnancy, follow-up sonography should be obtained to document the appearance of the yolk sac or embryo.44,45



As the sac enlarges, it gradually impresses and deforms the central cavity echo complex, giving rise to the characteristic sonographic appearance referred to as the double decidual sac sign (see Figs. 6-7 and 6-14).46,47 This sign, which is universally present when the MSD is 10 mm or greater, consists of two concentric echogenic lines surrounding a portion of the gestational sac. The line closest to the sac represents the combined smooth chorion-decidua capsularis, whereas the adjacent, more peripherally located line represents the decidua parietalis. The uterine cavity is the potential space between these two lines and often contains a trace of fluid. The double decidual sac sign is most effective with transabdominal sonography performed at 5 to 6 weeks’ GA because, using this approach, sonographers can confirm the presence of an IUP before a yolk sac is visualized. With the advent of nearly universal transvaginal sonography, the double decidual sac sign has been relegated to a lesser role. The sign may still be useful, however, in patients in whom transvaginal sonography is not possible.




Blood Flow in Early Pregnancy


The characteristic first trimester main uterine artery waveform obtained at the uterocervical junction consists of high-resistance flow with a prominent diastolic notch, although the notch will occasionally be absent in a normal patient (Fig. 6-15). The diastolic notch typically disappears during the second trimester and in some cases as early as 13 weeks’ gestation.21 Persistence of the diastolic notch into the third trimester correlates with umbilical cord and placental abnormalities.48



Doppler signals can also be obtained from the spiral arteries or subchorionic vessels, located at the junction of the myometrium and hyperechogenic choriodecidual tissues. Flow in these vessels is typically pulsatile, with a low-resistance pattern (Fig. 6-16). Deep to the subchorionic vasculature is the intervillous space (Fig. 6-17). In the first trimester, Doppler interrogation at this level reveals a more venouslike flow pattern, which may be difficult to detect (even in a more developed placenta) because of its extremely low velocity.22




Over the course of the first trimester, vascular impedence decreases and blood flow velocity increases, and the uteroplacental hemodynamics change from high resistance low volume to the low resistance high velocity state that will continue for the remainder of the pregnancy.31


During first trimester sonography, prominent hypoechoic areas with visible venous flow are occasionally seen around the margins of a gestational sac (Fig. 6-18). These have been termed venous lakes. Unlike true intervillous flow, which is difficult to localize without sensitive Doppler settings, slow flow within these vascular spaces can often be appreciated with gray-scale sonography alone if high-gain settings are used. Because of its extremely low velocity, the flow is difficult to document with color or pulsed Doppler. If no attempt is made to evaluate for the presence of flow, these spaces may be incorrectly considered subchorionic hematomas. Whether these vascular spaces have any significance with regard to pregnancy outcome is uncertain; opinions range from benign to ominous.49,50 Therefore, when multiple lakes are seen, close sonographic follow-up is probably indicated.




Identifying the Yolk Sac


The yolk sac is the first anatomic structure identified within the gestational sac. Embryologically, this is the secondary yolk sac, but because the primary yolk sac cannot be detected by sonography, sonologists refer to this structure simply as the yolk sac. Using a transvaginal approach, it may be visible as early as the beginning of the 5th gestational week (MSD, 5 mm), and it is almost always seen by 5.5 weeks’ GA (MSD, 8 mm).51 Using a transabdominal approach, the yolk sac should be evident by 7 weeks’ GA, when the MSD is 20 mm.52 Because detecting a yolk sac unequivocally confirms that an intrauterine fluid collection represents an early IUP as opposed to a pseudosac associated with an ectopic pregnancy, it is important to optimize scanning parameters to ensure its visualization.53 Given a choice, the highest possible transducer frequency as well as harmonic imaging should be selected (Fig. 6-19). Detecting the yolk sac is also important for purposes of assigning GA, and to locate the contiguous embryonic disk and early cardiac activity.51,52,54,55



The yolk sac is spherical in shape, with a well-defined echogenic periphery and a sonolucent center. Its diameter increases steadily between 5 and 10 weeks’ GA to a maximal diameter of 5 to 6 mm5658; this corresponds to a CRL of 30 to 45 mm (Fig. 6-20 and Table 6-3).58 As the GA advances, the yolk sac separates and ultimately detaches from the embryo but remains visible within the shrinking chorionic cavity. Subsequently, its diameter decreases,57,58 and it may occasionally become somewhat irregular in contour.56 By the end of the first trimester, it is no longer detected by sonography, although, if searched for, it can be found at delivery.59



Table 6-3 Mean Diameter of the Secondary Yolk Sac by Week






























Gestational Age (weeks) Sonographic Diameter (mm±SD)
5 3.01 ± 0.75
6 2.99 ± 0.73
7 3.99 ± 0.86
8 4.72 ± 0.64
9 5.22 ± 0.63
10 5.89 ± 0.56
11 5.35 ± 0.87
12 4.34 ± 0.62

Adapted from Jauniaux E, Jurkovic D, Henriet Y, et al: Development of the secondary human yolk sac: Correlation of sonographic and anatomical features. Hum Reprod 6:1160, 1991.



Identifying the Embryo and Cardiac Activity


Using state-of-the-art vaginal transducers, the embryonic disk is detected initially as a subtle area of focal thickening along the periphery of the yolk sac (Fig. 6-21). Most authorities agree that the threshold for embryo detection is when the disk measures 1 to 2 mm in length. Depending on the investigator, this corresponds to between 5 and 6 weeks’ GA6063 and a MSD of between 5 and 12 mm.6466



Embryologic investigation suggests that cardiac contractions begin at 36 to 37 days GA.12 Support for this is found in a series of patients who conceived with assisted reproduction. In these patients, transvaginal sonography detected embryonic cardiac activity at 34 gestational days, with a simultaneous embryonic length of 1.6 mm.67 Because the human eye is exquisitely sensitive to motion, very early cardiac pulsations can occasionally be appreciated by real-time sonography before the embryo itself is identified. For practical purposes, many sonologists consider the identification of cardiac activity in an embryo with a CRL of less than 5 mm as 6 weeks’ GA.68 Cardiac activity should be detected routinely when the embryo attains a length of 4 to 5 mm.67,69,70 This corresponds to a GA of 6.0 to 6.5 weeks, at which time the MSD is 13 to 18 mm.51,65,67,71,72 Using a transabdominal approach, cardiac activity should be evident by 8 weeks’ GA, when the MSD is 25 mm.52


During the first trimester, cardiac rates, which should be recorded using M-mode, vary with GA, but not with the sex of the embryo (Table 6-4 and Fig. 6-22).7277 When activity is obtained before 6 weeks, the rate is relatively slow, typically between 100 and 115 beats per minute (BPM) (Fig. 6-23).72,7880 Thereafter, it increases rapidly, and by 8 weeks is between 144 and 170 BPM.7274 After 9 weeks’ GA, the rate plateaus at 137 to 144 BPM.72 Examining an individual embryo for 15 to 60 minutes reveals almost no variation in cardiac rate75; in contrast, comparing embryonic heart rates at different GAs reveals progressively more variation in heart rate with increasing GA.76


Table 6-4 First-Trimester Fetal Heart Rates






























Gestational Age (weeks) Mean Fetal Heart Rate (beats per minute ± 1 SD)
5–5.95 101.2 ± 8.7
6–6.95 124.5 ± 12.1
7–7.95 128.0 ± 11.7
8–8.95 144.3 ± 19.5
9–9.95 138.7 ± 12.4
10–10.95 136.9 ± 10.9
11–11.95 139.8 ± 18.9
12–12.95 137.3 ± 12.9

Adapted from Hertzberg BS, Mahony BS, Bowie JD: First trimester fetal cardiac activity: Sonographic documentation of a progressive early rise in heart rate. J Ultrasound Med 7:573, 1988.




Sonographic observation throughout the embryonic period (weeks 6 to 10) reveals dramatic transformation of anatomic structures (see Figs. 6-6, 6-24, and 6-25 and Table 6-2). CRL length increases by approximately 1 mm/day.66,81 During the 6th week of development, with ventral folding of the cranial and caudal ends of the embryo, it changes rapidly from a flat disk into a 3D C-shaped structure.11 The rapidly developing brain and head become prominent as the rostral neuropore closes, and the caudal neuropore elongates and curves into a tail. Soon thereafter, as the amniotic sac surrounds the developing embryo, the yolk sac, and embryo diverge from one another. Despite the extra-amniotic location of the yolk sac, initially it remains attached to the embryo via the sonographically visible vitelline duct (also called the omphalomesenteric duct) (Fig. 6-26).82 This structure contains an artery and vein that transport blood elements, nutrients, and primordial sex cells from the yolk sac to the embryo. Between 7 and 8 weeks, limb buds evolve into paddle-shaped upper and lower limbs, with early development of the hands and feet. By the 9th week, the extremities protrude ventrally, the trunk begins to elongate and straighten, and midgut herniation into the umbilical cord is prominent. The 10th week (embryo length, 30 to 35 mm) reveals a distinctly human-appearing embryo, with visible and relatively opposed hands and feet and the tail no longer present (Fig. 6-27). Although a prominent embryologic textbook suggests return of the midgut to the abdomen during the 12th gestational week,18 sonographic analysis suggests this event is completed by the end of the 11th week.8385



image

FIGURE 6-25 Two-dimensional sonographic images that depict changes during the first trimester in embryonic size and shape. A. At 6.5 weeks’ gestational age (GA), it is not possible to differentiate the crown from the rump (embryo is measured between calipers) (compare with Fig. 6-6A). B. At 8 weeks’ GA, the embryo has a C-shaped configuration (compare with Fig. 6-6B). The cephalic end becomes prominent and contains an easily visible sonolucent structure: the developing rhombencephalon. The rump may reveal a tail-like appendage. Because of embryonic curvature, length measurement is often a neck-rump length (between calipers) rather than a crown-rump length. C. By 10 weeks GA (CRL of 30 mm between calipers), the limbs are becoming visible (open arrow), and the gut is herniating into the base of the umbilical cord (arrow) (compare with Fig. 6-6C). Note that the head is disproportionately large relative to body length. D. At the end of the first trimester, the fetus has a human-like appearance (compare with Fig. 6-6D). Note the relative proportionality between the head size and overall body length.


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