Sonography of the pregnant patient can reveal a wide variety of conditions, whether they cause acute discomfort or are asymptomatic. The greatest benefits of sonography over other diagnostic modalities in such a patient include its safety, low cost, and ability to localize and characterize pathology.

Gravid and nongravid women alike can be affected by uterine fibroids, dermoid cysts, physiologic cysts, and neoplasms. These can present as abdominal emergencies even though symptoms of pain and bloating can easily mimic normal discomforts of pregnancy. Imaging of the pregnant woman is most accurately and intuitively interpreted in the first trimester, before the growing uterus shifts abdominal anatomy. At any gestational age, sonography remains a superior diagnostic modality to distinguish the benign from the potentially dangerous without the harms of radiation and provides aid in management and therapeutic planning.

For many women, the sonographic examinations performed during pregnancy may be their first imaging studies. Previously asymptomatic or small pathology hidden to palpation may reveal itself to sonography and impacts subsequent clinical decisions regarding the pregnancy. Incidental masses may require prompt treatment, alter the labor and delivery plan, or necessitate further imaging. Recent joint guidelines on obstetrical sonography published by the American College of Radiology appropriately reflect the need for a comprehensive first trimester sonogram that includes the “uterus, cervix, adnexa, and cul de sac region” along with the gestational contents.¹ This ensures that any poorly localized symptoms are not mistaken for the normal discomforts of pregnancy.

Undetected nonobstetrical abnormalities can cause significant complications despite their frequently benign cytology. The hormonal effects of pregnancy and increasing uterine girth can cause leiomyomata to enlarge, cysts to rupture, adnexal masses to undergo torsion, and cancers to grow. Early identification of abnormalities in the first trimester facilitates surgical treatment, if necessary, during the second trimester. At that time, risk of spontaneous abortion and preterm labor are lowest, and surgical exposure remains adequate. Though smaller incidental masses with benign sonographic characteristics are amenable to observation, surgical intervention is typically initiated for those that are larger (usually >7 cm in diameter), undergoing torsion, and/or suspicious for malignancy.² Sonography can be used to accurately differentiate between the architectural patterns of benign and malignant masses and to determine which would be associated with an increased risk of ovarian torsion.^3-6

Identification of incidental findings on transabdominal obstetrical sonography, inability to visualize the adnexa or cervix, or examination of an obese patient may obligate further study via the transvaginal approach.^1,3 This technique is generally quite tolerable for the patient and avoids signal attenuation by subcutaneous tissues. In addition, it provides higher-resolution views of pelvic pathology owing to probes of higher frequency that contain anatomy of interest within a shallower focal length than commonly used transabdominal probes.

Three-dimensional sonography has proven particularly beneficial in imaging of the uterus and adnexa. Transvaginal probes have been adapted to collect many consecutive two-dimensional images throughout a region of interest while the probe is held stationary. This creates a user-independent, life-like volume that can be manipulated and reconstructed in the coronal plane. This plane cannot usually be obtained by two-dimensional transvaginal sonography but can add additional information that is essential when evaluating uterine anomalies.

Both two-dimensional and three-dimensional sonography can be accompanied by color Doppler sonography. Evaluation of incidental findings must consider the local vascular tree, though physiologic hemodynamic changes of pregnancy can complicate analysis. In general, disorganized vasculature with low resistance and high flow is characteristic of ominous diagnoses.³ Early in the first trimester, embryos are most susceptible to external teratogens, which theoretically includes the thermal and mechanical energy generated by pulsed spectral Doppler, in particular.⁷ Thus, the American Institute of Ultrasound in Medicine recommends utilization of Doppler studies that include the embryo or fetus only when there is clear diagnostic benefit while minimizing embryonic exposure time and intensity.⁸

Fibroids

Leiomyomas, benign smooth muscle tumors commonly referred to as “fibroids,” are the most prevalent gynecologic affliction of the gravid and nongravid female. They are commonly found incidentally on first trimester sonography,^4,5 and they may complicate pregnancies later in their course through mass effect if the fibroids are >300 cm³ (ie, miscarriages, growth restriction, obstruction to vaginal delivery).²² These persistent, round, well-defined masses are iso- or slightly hypoechoic compared to the surrounding myometrium and demonstrate peripheral vascularity by color Doppler sonography. They may contain shadowing calcifications and areas of cystic change when undergoing degeneration (Figure 32-1).

Fibroids are highly sensitive to estrogen, which can promote their growth and maturation, particularly in the first trimester. They may even grow so large as to overwhelm their blood supply, resulting in acutely painful degenerative changes including changing echogenicity and loss of clear circumferential vascularity on sonograms. The loss of blood supply may result in various types of degeneration: hyaline or myxoid degeneration, calcification, cystic degeneration, or red (hemorrhagic) degeneration. Red, or carneous, degeneration is a hemorrhagic infarction secondary to venous thrombosis within the periphery of the tumor or rupture of intratumoral arteries. Degeneration more commonly occurs within the weeks following delivery or later in pregnancy, as the uterine wall grows, sometimes stretching vessels and occluding blood flow to fibroids.²³

It is of upmost importance to utilize sonography early in pregnancy to identify those fibroids that would be clinically significant due to their size and location. Submucosal fibroids may increase the risk of early pregnancy loss. If first trimester miscarriage is eluded, these benign masses can have significant detrimental ramifications that are not realized until later trimesters. Mass effect and disruption of placental implantation caused by large fibroids competes with fetal growth and can obstruct fetal and placental delivery if located within the lower uterine segment.^5,9 Increased pressure above a low-lying fibroid during labor increases risk of uterine rupture and fetal morbidity and mortality. Despite these complications, intervention is usually not necessary or commonly pursued until the postpartum period.

The pervasive fibroid can present unexpected challenges for the medical imaging specialist. Subserosal-type fibroids pushed close to an ovary by the gravid uterus can be difficult to differentiate from a solid ovarian mass. Degenerative changes in such a fibroid may further complicate diagnosis. Imaging a separate ovary, often better differentiated on three-dimensional sonography, will exclude an ovarian mass. Color Doppler may also be helpful in delineating blood flow that connects the uterus and fibroid. If sonography is inconclusive, further imaging with MRI may be required.

Traditional management of adnexal masses during pregnancy has been surgical, but with surgeries come both fetal and maternal risks. One of the major goals of sonographic evaluation is to determine when conservative management with observation is appropriate. Simple cysts of any size and classic-appearing hemorrhagic cysts are highly unlikely to be malignant lesions in menstruating females. Follow-up or further evaluation is recommended only when the size is greater than 5 cm. Reports have shown a high accuracy of sonographic determination of malignant potential of ovarian masses found in pregnant patients. Schmeler et al and Kumari et al reported correct diagnosis of malignancy in all pregnant patients studied presenting with incidental adnexal masses.^6,10

Corpus Lutea

A retrospective review of sonography performed on over 18,000 pregnant patients identified a 2.3% prevalence of adnexal masses; the majority were small (<5 cm), simple cysts that were without complication during the pregnancies.¹¹ The majority of these cysts likely begin as corpora lutea, the most commonly encountered cystic adnexal mass during pregnancy.⁴ Corpora lutea form after fertilization of an expulsed ovum from an ovarian follicle. They endure to produce progesterone and maintain the early pregnancy. Fluid-filled with smooth, thick walls, they grow to a maximum diameter at the end of the first trimester. The decreasing functionality of the corpus luteum as the placenta assumes an endocrinologic role is reflected sonographically by its serially shrinking size and involution by the second trimester.

The lifetime of the corpus luteum in a pregnant woman is much longer than during a normal menstrual cycle, and it has more opportunity to grow; thus, complications such as rupture, torsion, and hemorrhage can more commonly occur in a pregnant patient (Figures 32-2A and B). However, intervention and further imaging are unnecessary for this physiologic incidental finding and should not be pursued during the first trimester when progesterone production is essential.

Corpus Luteum Cysts

A persistent corpus luteum can seal externally within the ovary and continue to collect fluid within, forming a unilocular corpus luteum cyst. Because the cyst contains fluid, it is anechoic with enhanced through transmission, though it may exhibit thin lace-like echogenic septae representing fibrin strands if it persists into the second trimester and is filled with blood.⁵ The size of the cyst is a strong predictor of its ability to spontaneously regress, with almost all cysts under 5 cm in diameter resolving completely without intervention.¹² The most recent guidelines (2010) for nongravid women from the Society of Radiologists in Ultrasound do not recommend follow-up sonography for simple cysts smaller than 5 cm, whereas yearly sonographic examinations of larger cysts should be considered, despite low malignant potential.¹³ Standard scheduling of obstetric ultrasounds offers the opportunity to track the growth of corpus luteum cysts throughout pregnancy.

Both the corpus luteum and corpus luteum cyst have distinguishing dense peripheral “ring of fire” vascularity on color Doppler imaging (Figures 32-3A through C). These vessels exhibit low resistance and high diastolic flow on spectral Doppler. There are typically little or no internal solid components. Ectopic or heterotopic pregnancies in the adnexa imitate corpus luteum cysts because they, too, are fed by a peripheral ring of vessels and can be seen directly adjacent to a cyst (Figures 32-4A and B). The critical distinction is made by determining if the adnexal mass is para- or intraovarian. Ectopic pregnancies should move independently from the ovary with pressure applied by the examiner. This “sliding sign” is not visualized during examination of intraovarian corpus luteum cysts, which remain coordinated in movement with the ovary. In a retrospective study of 78 pelvic sonograms performed on women exhibiting symptoms consistent with ectopic pregnancy during the first trimester, the radiologists were able to correctly identify ectopics in 23 of 27 patients exhibiting the “sliding organ sign.” Although not a strong differentiator, ectopic pregnancies also tend to be more complex and echogenic than luteal cysts when compared to the ovarian parenchyma.¹⁴

Corpus luteum cysts are usually asymptomatic, especially when they are relatively small in size, as opposed to ectopic pregnancies that will invariably become symptomatic. However, large cysts can rupture, undergo torsion, and bleed.¹² Intervention is imperative for ectopic pregnancies and recommended for cysts and benign masses greater than 7 cm, but not recommended for small luteal cysts.¹³

Hemorrhagic Corpus Luteum Cysts

The clinical presentation of a hemorrhagic corpus luteum cyst is characterized by more unilateral pain than its predecessors. The resolution of pain does not correlate with resolution of the hemorrhagic cyst, which can evolve over subsequent months.¹⁵ Sonographically, the acute phase of the hemorrhage demonstrates very hyperechoic internal echoes (Figures 32-5A and B). As the blood settles, the cyst appears more heterogeneous with thin, fibrinous septations that are without color Doppler flow. The clot retracts to the walls of the cyst, appearing as a solid or reticular hyperechoic structure. Throughout this course, the cyst should always remain well defined with enhanced through-transmission owing to the predominant presence of nonbloody cystic fluid. If the cyst is not intact and the patient is symptomatic, a diagnosis of rupture is supported by the presence of free pelvic fluid.

Due to the lack of specificity observed in some hemorrhagic corpus luteum cysts, follow-up imaging may be appropriate. Growth requires continued surveillance. The presence of thick septations and nodular walls, especially when there is associated vascularity, is suspicious for neoplasia, and surgical interventional must be considered. Alternatively, magnetic resonance imaging may be helpful for further characterization (Figures 32-6A through C). By the second trimester anatomy scan, true functional hemorrhagic cysts should have involuted.