Early Options: Ultrasound Imaging in Early Pregnancy

Early Options - A Provider's Guide to Medical Abortion

This page contains the following sections:

Principles of Sonographic Imaging
Interpreting the Sonographic Image
Transabdominal Versus Transvaginal Ultrasound
Transvaginal Ultrasonography: Method
Ultrasonographic Landmarks of the Nonpregnant Uterus
Identifying Early Pregnancy
Yolk Sac and Embryo
Gestational Age Formulas

Diagnostic ultrasound is a versatile and safe imaging modality with many applications in the field of obstetrics and gynecology. Ultrasound technology permits the early diagnosis of pregnancy and has significantly advanced both abortion and prenatal care. In the context of medical abortion, ultrasonography can help determine gestational age, assess the outcome of the procedure, and diagnose ectopic pregnancy and other types of abnormal pregnancy.

Principles of Sonographic Imaging
Sound is a waveform of energy. Ultrasound employs high-frequency sound waves in the range of 2 to 10 MHz to create a visual image.²⁰ The ultrasound probe generates sound waves capable of penetrating organic tissue and also serves as a receiver of sound wave energy. Reflected waves (echoes) are detected by a transducer within the probe and converted to an electrical signal. This signal is used to generate a two-dimensional image on an oscilloscope (the ultrasound monitor).

Interpreting the Sonographic Image
Visual orientation on the ultrasonographic monitor can be a challenge for first-time users. The customary view "hangs" the pelvic structures from the top of the screen. With a longitudinal (or sagittal) orientation, the top of the uterus lies inferior to the cervix, the left side of the screen is anterior, and the right side is posterior. Therefore, the fundus of an anteflexed uterus will appear on the left side of the monitor screen and that of a retroflexed uterus will appear on the right.

To generate a transverse (or coronal) orientation, the sonographer turns the probe 90 degrees counterclockwise. This maintains the right side of the patient on the left of the monitor screen, in keeping with the standard conventions of medical imaging.

After reading this section, you should be able to answer the following questions:
Which characteristics do transabdominal and transvaginal ultrasound share?
What are the features of transabdominal ultrasound?

Transabdominal Versus Transvaginal Ultrasound
Transabdominal ultrasound (TAU) and transvaginal ultrasound (TVU) are valuable diagnostic tools in obstetrics and gynecology. There are advantages and disadvantages to both methods. Click here to view Figure 1.

Transabdominal ultrasound provides a panoramic view of the abdomen and pelvis and is noninvasive, whereas TVU provides a more limited pelvic view and requires insertion of a probe into the vagina. Click here to view Figure 2.

The image generated by transvaginal ultrasound provides a better view of the uterus and adnexa during early pregnancy. The transabdominal study requires a full bladder for optimal visualization of pelvic structures. This can be accomplished by asking the patient to drink 6 glasses of water about 30 minutes before the ultrasound examination. Some patients find TVU more comfortable than TAU because TVU does not require a distended bladder.

For a transabdominal study, patients do not necessarily need to undress, although a drape placed on the lower torso can be helpful to protect the patient's clothing from the ultrasound gel. The exam is conducted in the dorsosupine position. For a transvaginal sonogram, patients are undressed from the waist down and assume the lithotomy position.

Both TVU and TAU are easy to learn and can be readily combined with pelvic examination, although the expense of either ultrasound study may be an important consideration in some clinical settings.

Transabdominal ultrasound cannot reliably diagnose pregnancies that are < 6 weeks' gestation. Transvaginal ultrasound, by contrast, can detect pregnancies earlier, at approximately 4 ½ to 5 weeks' gestation. Prompt diagnosis made possible by TVU can, therefore, result in earlier treatment.

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Transvaginal Ultrasonography: Method
As part of the counseling process, patients who will undergo sonography should be informed about the nature of the examination, how to prepare for it, and what to expect.

For the transvaginal scan, ultrasound gel is applied to the disinfected transvaginal probe tip before applying the probe cover. Next, the probe tip is lubricated. The probe is then inserted into the vagina in the longitudinal axis and aimed at the uterus.

A marker on the probe identifies the axis of the view that will appear on the monitor. This marker should be pointed up to visualize the longitudinal axis. By moving the probe horizontally, the sonographer identifies the uterus, endometrial stripe, pregnancy (if present), and cul-de-sac. Next, the user rotates the probe 90 degrees counterclockwise for the transverse orientation in order to examine the uterus and adnexa in a series of cross-sectional "slices."

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Ultrasonographic Landmarks of the Nonpregnant Uterus
The uterus appears on the ultrasound monitor as a pear-shaped organ, homogeneous in appearance, with an increased echogenic (white-appearing) layer corresponding to the endometrium. This echogenic tissue layer is often referred to as the endometrial stripe.

With TAU, the fluid-filled bladder is anechoic (black-appearing) and readily visible. In contrast, with TVU, the empty bladder is difficult to visualize. Bowel motility can often be seen, particularly with transvaginal imaging.

After reading this section, you should be able to answer the following questions:
At what point should experienced ultrasonographers be able to identify a gestational sac with transvaginal ultrasound?
The "double-ring sign" usually appears at 5½ to 6 weeks' gestation. This sign helps distinguish a true gestational sac from what other possibilities?

Identifying Early Pregnancy
The earliest definitive evidence of pregnancy visible on ultrasonography is the gestational sac. In experienced hands, it may be detected as early as 30 days' gestation by TVU. It initially appears as a round, anechoic structure.

At this early stage of pregnancy, the gestational sac already contains the yolk sac, embryonic disk, and amnion, but these structures are too small to be visualized by sonography.²¹ When it first appears on ultrasonic imaging, the gestational sac is surrounded by a thickened decidua. This perimeter then becomes a distinct "double ring" (also known as the "double decidual sign"). The inner ring corresponds to the decidua capsularis and the chorion laeve, and the outer ring corresponds to the decidua vera.²²

The double ring usually appears by 5½ or 6 weeks' gestation. This finding is helpful in distinguishing a true gestational sac from an intrauterine fluid collection, or pseudosac. Click here to view Figures 3A and 3B. In the setting of an ectopic pregnancy, misinterpreting a pseudosac as a true gestational sac could lead to an incorrect diagnosis of a normal intrauterine pregnancy.²³

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There is a predictable correlation between serum ß-hCG levels and sonographic findings. The "discriminatory level" is defined as that ß-hCG level at which all normal gestational sacs should be visualized.

For transabdominal ultrasound performed by an experienced sonographer, the established discriminatory level is approximately 3,600 mIU/mL.^24,25 With TVU, the discriminatory level is both operator- and equipment-dependent. The sac may be seen at ß-hCG levels as low as 800 mIU/mL, but should be identified by experienced ultrasonographers at 2,000 mIU/mL.¹⁷

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Yolk Sac and Embryo
After the gestational sac, the next gestational milestone to appear is the extraembryonic yolk sac. Click here to view Figure 3C. It can be identified at approximately 5 to 6 weeks' gestation and persists until about 10 weeks' gestation.

An experienced sonographer can detect a yolk sac with transvaginal ultrasound when the gestational sac has reached a mean diameter of 8 mm to 10 mm.²⁶ The presence of a yolk sac confirms the diagnosis of an intrauterine pregnancy and excludes ectopic pregnancy, except in rare cases of simultaneous intrauterine and extrauterine gestations.

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The embryo is first imaged transvaginally at about 6 weeks' gestation as a hyperechoic linear structure referred to as an "embryonic pole." Click here to view Figure 3C. In normal pregnancy the embryo grows about 1 mm a day, reaching a length of 7 mm by approximately 49 days' gestation. With transvaginal ultrasonography, cardiac motion can sometimes be seen in a 2-mm to 3-mm embryo, and is invariably detected in normal pregnancy when the length of the embryo reaches 5 mm.

After reading this section, you should be able to answer the following question:
How is gestational age calculated from sonographic scans?

Gestational Age Formulas
Ultrasound can provide accurate estimates of gestational age during early pregnancy. Initially, sonographic dating is based on the size of the gestational sac. Electronic calipers may be used to measure the inner margins of the gestational sac in two or three dimensions. These measurements are used to determine the mean sac diameter.

Modern ultrasound machines have built-in software to calculate gestational age based on these measurements. The tables used to generate this estimate vary among machines, so clinicians should recognize this potential source of variability in dating. Conversion charts equating measurements with gestational age can also be found in radiology and obstetrics-gynecology textbooks.

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Many formulas exist for estimating gestational age based on sac measurement. In 1988, Rossavik and colleagues²⁷ developed the following formula based on mean sac diameter: gestational age (days) = mean sac diameter (mm) + 30 ± 3 days.

Once the embryo becomes visible, the greatest length of the embryo provides the data on which to base the estimate of gestational age. Prior to approximately 49 days, the embryo appears linear, and the measurements of this structure (the embryonic pole) are straightforward.

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Soon after 49 days, the embryo assumes a curved shape, and the greatest linear dimension of the embryo becomes the crown-rump length. A reliable formula developed by Goldstein and Wolfson²⁸ correlates embryonic length and gestational age: gestational age (days) = embryonic length (mm) + 42 ± 3 days.

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Proceed to Monitoring Medical Abortion.

References for this module