A pelvic or transvaginal ultrasound is a non-invasive imaging technique that uses high-frequency sound waves to create live images of the pelvic organs. This technology relies on echogenicity, where different body tissues reflect sound waves back to the probe at varying rates. This process allows medical professionals to visualize the size, shape, and internal structure of the uterus and surrounding organs, providing a detailed cross-sectional view of the muscular wall and inner lining.
Establishing the Baseline: A Normal Uterus Scan
A healthy uterus typically presents as a smooth, pear-shaped structure on an ultrasound image. The muscular wall, known as the myometrium, should exhibit a homogeneous or uniform texture, meaning its appearance is consistent throughout. This muscle layer is generally divided into three distinct sonographic zones: a thin, inner hypoechoic layer, a thick, middle homogeneous layer, and a thin, outer layer.
The innermost layer, the endometrium, is the lining of the uterine cavity and its appearance changes dramatically based on the menstrual cycle phase. Immediately following menstruation, the endometrium is visible as a thin, bright, central line. As the cycle progresses toward ovulation, the lining thickens and develops a characteristic trilaminar or “triple-stripe” appearance.
After ovulation, the endometrial stripe continues to thicken and becomes more uniformly bright or echogenic. The normal thickness of this lining varies significantly, ranging from approximately 1 to 14 millimeters, depending on the hormonal phase. Any deviation from this expected shape, uniform texture, or cyclical thickness defines an abnormal uterine ultrasound.
Identifying Localized Growths and Masses
Abnormalities often manifest as discrete, localized masses that disrupt the smooth structure of the normal uterus. Uterine fibroids, or myomas, are solid, benign tumors arising from the myometrium and are the most common localized growth seen on ultrasound. These masses typically appear as well-defined, round or oval structures that are hypoechoic, or darker, compared to the surrounding muscle tissue.
The visual characteristics of a fibroid depend heavily on its location within the uterine wall. Intramural fibroids are embedded entirely within the muscle, while subserosal fibroids protrude from the outer surface of the uterus. Both types often cause a phenomenon called acoustic shadowing, which is a dark area behind the mass caused by the sound waves being blocked by the dense tissue.
Submucosal fibroids grow directly beneath the endometrial lining and project into the uterine cavity, often causing the most symptoms. These masses must be distinguished from endometrial polyps, which are growths of the lining itself. Polyps typically present as bright, homogeneous masses attached by a stalk, usually supplied by a single feeding artery visible with color Doppler imaging. In contrast, submucosal fibroids are often darker, distort the interface between the lining and the muscle, and show multiple surrounding vessels.
Abnormalities of the Uterine Wall and Endometrial Lining
Some conditions cause diffuse changes across the entire uterine structure rather than forming a discrete mass. Adenomyosis is a condition where the tissue that normally lines the uterus grows into the myometrial muscle wall. The uterus often appears globular and symmetrically enlarged, losing its typical pear shape. The myometrium itself becomes heterogeneous, meaning its texture is mottled and inconsistent, due to misplaced tissue and muscle overgrowth.
Other common features of adenomyosis include small, fluid-filled pockets called myometrial cysts within the muscle wall. An irregular or indistinct junctional zone, the border between the endometrium and the myometrium, is also a strong indicator of this diffuse abnormality.
Endometrial hyperplasia involves a thickening of the uterine lining beyond the expected range for the menstrual cycle phase. This thickening is considered abnormal when the stripe measurement is significantly greater than the established cutoff value, such as greater than 4 millimeters in a postmenopausal woman not taking hormones. On ultrasound, the thickened lining typically appears bright and may have an irregular outline. Accurate measurement and correlation with the patient’s clinical status and hormonal use are necessary for this diagnosis.
Variations in Uterine Shape
Congenital variations in the overall shape of the uterus stem from incomplete fusion during fetal development. These structural differences are apparent even if the uterine tissue itself is otherwise healthy. A septate uterus is characterized by a normal, convex or flat external contour, but the interior cavity is divided by a wall of fibrous or muscular tissue called a septum.
This septum extends down from the top of the uterus, creating two separate halves of the uterine cavity. The tissue making up the septum usually appears isoechoic, or similar in brightness, to the surrounding myometrium.
In contrast, a bicornuate uterus, commonly described as heart-shaped, features an indentation or cleft on the external surface of the fundus. This anomaly results in two distinct uterine horns that diverge from a common lower segment. For diagnosis, this external indentation must typically measure greater than one centimeter deep. The key difference is that a septate uterus has a normal exterior shape, while a bicornuate uterus has a visibly abnormal external contour.