Abstract
Transvaginal ultrasound scan (TVS) is the recommended method for assessing pregnancies in the first trimester. Transabdominal scanning can be performed if women are reluctant to have a TVS, but the limitations of this method over TVS should be explained. Examination with TVS provides a detailed evaluation of the endometrial cavity and ovaries, but the high-frequency transducer that allows improved near-field resolution compared with transabdominal examinations suffers from limited sound penetration (far-field imaging). The vaginal probe, due to its proximity to the uterus, allows excellent views of the uterus and pelvis in order to view and measure the gestational sac and any embryonic structures, which are small in the first trimester.
Introduction
Transvaginal ultrasound scan (TVS) is the recommended method for assessing pregnancies in the first trimester [1]. Transabdominal scanning can be performed if women are reluctant to have a TVS, but the limitations of this method over TVS should be explained. Examination with TVS provides a detailed evaluation of the endometrial cavity and ovaries, but the high-frequency transducer that allows improved near-field resolution compared with transabdominal examinations suffers from limited sound penetration (far-field imaging) [2,3]. The vaginal probe, due to its proximity to the uterus, allows excellent views of the uterus and pelvis in order to view and measure the gestational sac and any embryonic structures, which are small in the first trimester.
The main purpose of the early pregnancy scan is to locate the pregnancy, assess the viability and gestational age and establish the number of pregnancies. It is important to consider maternal anatomy also, as this may aid determining between an intrauterine pregnancy and an ectopic pregnancy [1,3]. This assessment can be achieved using four key movements [3], though sliding and panning is restricted when performing a vaginal scan (Table 14.1).
Table 14.1 Movements possible while scanning early pregnancy using the transvaginal approach
| The transducer should be moved gently and slowly: |
| Sliding refers to moving the transducer along the length of the vagina. |
| Rotating is to move the transducer 90 degrees along its long axis (from the usual position with the groove at the 12 o’clock to 9 o’clock position). |
| Rocking or tilting of the transducer is a motion made along the long axis of the probe with the transducer array held stationary. |
| Panning is moving the entire transducer in one plane (horizontal or transverse). |
While scanning techniques are described in Chapter 1, this is further discussed here. When beginning to scan, it can be difficult to correlate the movement of the probe with the organs you wish to image. An ordinary pear may serve as an excellent example (Figure 14.1). If you hold the pear in the palm of your hand, with the narrower end pointing towards the wrist, you have in your hand a representation of the uterus, with the broader part of the pear representing the fundus and the narrower end, the cervix.
Figure 14.1 Longitudinal views through the pear, as an analogy to scanning through the uterus in a sagittal section.
When you insert the probe into the vagina, the first view you are trying to obtain is in the sagittal view (Figures 14.1 and 14.2). Hopefully you can see the similarity. If we were to continue to slice the pear evenly, as a loaf of sliced bread, we would create a 3D representation of the ultrasound ‘slices’ that our imaging produces. Similarly, if we slice a pear across its widest circumference and continue to slice evenly like sliced bread (Figure 14.3), we create a representation of the uterus in transverse views (Figure 14.4).
Figure 14.3 Transverse sections through the pear.
An intrauterine pregnancy will be visualized within the endometrium eccentric to the midline, which is the bright white horizontal line in Figure 14.4.
Systematic Approach
When beginning to scan, it is important to develop a systematic approach in order not to miss any pathology. It is easy to get distracted by an embryo, which catches the operator’s attention, and then easily forget to confirm the location of the pregnancy.
Once the sagittal section of the uterus has been obtained, the next step is to scan through the uterus from side to side. Without causing too much discomfort to the patient, it is possible to build an image of the uterus and confirm the location of the gestational sac (Figures 14.5 and 14.6). It is helpful to see the fundus and cervix in the same plane as the sac, as this will help avoid missing an interstitial pregnancy, which will be seen laterally to the midline. Rotate the probe anticlockwise by 90 degrees to move from the longitudinal to transverse view.
Figure 14.5 Intrauterine pregnancy.
Figure 14.6 Laterally placed gestational sac.
Once the surveillance in the longitudinal plane is completed, move the probe back to the midline of the patient, and rotate the probe anticlockwise 90 degrees and tilt the probe upwards and downwards in a gentle motion to view the uterus in transverse. An image of the largest cross-section of the uterus, which features the endometrium and the outline of the uterus, will be almost oval in shape (Figure 14.4). By tilting the probe upwards, the fundus will come into view in an anteverted uterus, and by moving the probe down the cervix will come into view. In a retroverted uterus the reverse is true: moving upwards will bring the cervix into view and angling the probe downwards will bring the fundus into view. If the uterus is very anteflexed or retroflexed it may be difficult to complete the scan in this plane due to the limited space in the vagina.
When measuring the gestational sac, the same principles should be followed as for measuring any cystic structure. The sac should be measured in three orthogonal planes: at the maximum longitudinal diameter, the maximum anterior–posterior diameter and the maximum transverse diameter in transverse view.
Structures Seen in Normal Intrauterine Pregnancy
The gestational sac is the first definitive sign of an intrauterine pregnancy, which can be seen as early as the fourth week, but generally is clearer from the fifth. Before such structures are visible, the endometrium is thickened and has a hyperechoic appearance due to exposure to progesterone. The gestation sac is filled with clear fluid and therefore does not reflect ultrasound. As such, it is described as anechoic, which refers to the black appearance on ultrasound of the inside of the gestational sac. This actually consists of two fluid-filled compartments – the inner amniotic cavity and the outer chorionic cavity. The outer edge of the gestational sac, by contrast, is bright and echogenic and therefore hyperechoic in relation to the fluid within. This ring corresponds to the invading chorionic villi. Depending on length of a woman’s cycle, sometimes the sac cannot be visualized until after the fifth week. Due to uncertainty as to the conception dates in cases of recently stopping hormonal contraception, breast feeding and irregular cycle length, visualization of the gestation sac may not correspond with the patient’s dates. This should be considered when diagnosing pregnancies of unknown location and intrauterine pregnancies of uncertain viability, as well as ectopic pregnancies.
The bright echogenic rim eccentric to the midline is the distinguishing feature of a true gestational sac (Figure 14.7) versus a ‘pseudo sac’ (Figure 14.8). It should be noted that a failing intrauterine pregnancy may also have a thinner wall than a normal gestational sac. A word or two should be mentioned about the ‘pseudo sac’ within the context of ectopic pregnancy diagnosis. This is not a sac per se as it has no wall, but a small collection of non-clotted blood within the cavity. It can be differentiated from a normal gestational sac by the lack of a double echogenic ring around its anechoic contents. A normal gestational sac will also appear regular and circular in the longitudinal section, whereas the pseudo sac has a thin, not markedly as echogenic and more elongated appearance as it follows the shape of the uterine cavity (Figure 14.8) [3–5]. In a woman with a bicornuate uterus, a small amount of bleeding may be present in the empty hemi-uterus, which may also create a pseudo sac appearance. As with any skill requiring pattern recognition, the more normal gestational sacs one sees, the more likely one is to recognize when an anechoic structure within the cavity does not look entirely normal. Decreasing the gain on the machine (making the image darker) may help to highlight the decidual reaction surrounding a normal gestation sac, a feature that will be absent in a pseudo sac.
Figure 14.7 Intrauterine gestational sac – note the bright echogenic ring eccentric to the midline of endometrium.
Figure 14.8 Pseudo sac – note the collection of non-clotted blood in the endometrial cavity with irregular outlines and no decidual reaction.
By contrast to the black, circular appearance of the gestational sac, the yolk sac is seen as a bright echogenic ‘ring’-like structure within the gestation sac and may be visualized from around five weeks’ gestation. The foetal pole will appear adjacent to this from around six weeks (Figures 14.9 and 14.10). The foetal pole is hypoechoic compared to the yolk sac. Cardiac activity should be detectable once the crown–rump length (CRL) of the embryo measures 7 mm, though it can be, and most of the time is, detected much earlier, when the CRL measures 3 mm. Cardiac activity at this early gestation appears as a small echogenic equals sign (=), where the lines move together and apart in a pulsating manner.
It is worth bearing in mind that dates from the last menstrual period may be unreliable even when the woman reports a regular cycle, as an unusually early or late ovulation may occur. With the introduction of over-the-counter pregnancy dating tests and use of phone apps, there can often be confusion about dates and what these mean in correlation to what the scan reveals. Therefore, although it is important to have an idea of what we should expect to see on a scan based on this information, consideration should be given when interpreting our scan findings.
The gestational sac may occasionally implant in the lower part of the endometrial cavity and result in a normal pregnancy. It is necessary in these cases to differentiate a low implantation from a miscarriage in progress, a Caesarean scar ectopic pregnancy (Figure 14.11) or a cervical ectopic pregnancy. In the case of women who have had a previous Caesarean section, it is important to check that the gestational sac or trophoblastic tissue has not implanted into the scar itself. This is detailed later in this chapter.
Figure 14.11 Caesarean scar pregnancy with empty uterine cavity and gestational sac at the Caesarean scar site (arrow). Notice the proximity of the gestational sac to the bladder and thin layer of myometrium (star) between the sac and the brighter hyperechoic (thick arrow) posterior bladder wall. The uterine fundus has also been displaced posteriorly due to the sac site (and has not been included in this image).
The discriminatory hCG level refers to the level at which a gestational sac should be visible in a normal single intrauterine pregnancy in a normal uterus. This has traditionally been 1500 IU/L for a transvaginal scan and 6000 IU/L on transabdominal scan [6], but with the advent of high-resolution ultrasound machines the current discriminatory hCG level is much lower. Therefore, under no circumstances should scanning be delayed in the event that the hCG level is below an arbitrary cutoff of 1500 IU/L, as ectopic pregnancies do occur and rupture below this level. The presence of fibroids and the anatomy of the uterus will affect the ability to visualize an early pregnancy. The yolk sac can be seen when the mean sac diameter (MSD) of the gestational sac is between 5 and 12 mm. Its function is to transport nutrition to the embryo, haematopoiesis and storage of primordial germ cells [2].
Measuring the Crown–Rump Length
To illustrate the measuring of the embryo (or foetus after 10/40 gestation), an analogy can be made between a jelly baby (a humanoid-shaped sweet) and an embryo seen on scan. Holding the jelly baby longitudinally between one finger and the thumb and gently squeezing it to create a small curvature approximates a seven-week embryo complete with forebrain, heart bulge and rump. The white lines demonstrate how the measurement of the CRL can differ depending on the plane in which the embryo is viewed and thus the placement of the callipers (Figure 14.12). During a real scan in the very early gestation, these measurements differ only slightly, with no major impact on the actual due date (Figure 14.13). The crown and the rump are difficult to visualize at the early stages of pregnancy, so always obtain the longest measurement.
(a) Sagittal view of a jelly baby
(b) And a coronal view.
Figure 14.13 Sagittal and coronal view of a seven-week foetus. Measurements 1 and 2 are identical as they are measuring the same distance in two planes (sagittal on the left and coronal on the right); however, the more accurate CRL is represented by measurement 3 as this starts at the top of the crown. The difference in measurement is negligible, however. The crown is characterized by the telencephalon (hypoechoic area marked with an arrow). The images are enhanced by volume contrast imaging (VCI).
Figures 14.12 and 14.13 demonstrate the difference between sagittal and coronal views and how the coronal measurement of CRL may differ from the sagittal one. When the embryo only measures a few millimetres, a millimetre not measured can affect the dating of the pregnancy (Figures 14.14 and 14.15). However, with a TVS and often due to foetal position, it may not be possible to obtain a true sagittal section of the foetus to measure a ‘true’ CRL, hence a coronal plane may be all that is available. In this case a rescan in 7–14 days may be offered to more accurately date the pregnancy. As foetal structures are very small at this stage of pregnancy, only small movements of the transducer and mainly rotation along the long axis are necessary to produce an acceptable image of the foetus on scan and allow for accurate measurement. This is especially important when confirming miscarriage (see details in the subsequent sections). At 12 weeks’ gestation, foetuses, much like the children they may become, are not always well behaved and their position may inhibit measurement of a true CRL. Options are to wait for a foetus to move, ask the patient to change position (i.e. lift the pelvis up and down, tilt), or try an abdominal scan once it has been confirmed that the pregnancy is intrauterine.
Figure 14.14 Images taken during a single scan. Different calliper placements may alter the measurement of the CRL and affect dating of the pregnancy. Different planes obtained during the scanning may also add to the discrepancy.
Figure 14.15 Four-plus weeks’ gestation. Within an anteverted and anteflexed uterus a small gestation sac is visible (arrow). Note the brighter area surrounding it (decidual reaction). The endometrium is also thickened (callipers).
Dating of the pregnancy can be performed by using the inbuilt CRL reference charts (i.e. Hadlock or Chitty charts) on the scanning machine, or after the examination using a database software, where reports can also be written and stored. Figure 14.14 demonstrates the possible differences in measurements depending on the starting points and the obtained planes. The nuchal translucency (NT) scan may be booked based on the early pregnancy scan, so that the CRL on the actual day of the NT screening will be within the acceptable range of 45–84 mm, assuming a 1 mm per day elongation of the CRL. Changes in sonographic appearance of pregnancy and its development are demonstrated in Figures 14.15–14.23.
