21.3 Cervix & Congenital Uterine Anomalies

Cervical Anatomy and Benign Findings

The cervix is the cylindrical lower third of the uterus, normally about 2-4 cm long, extending from the internal os (at the isthmus) to the external os projecting into the vagina. On transvaginal imaging the cervical canal appears as a thin echogenic line running through a moderately echogenic muscular wall; transabdominally the cervix is best seen with a moderately full bladder.

Nabothian cysts are benign mucous retention cysts that form when squamous epithelium overgrows and obstructs endocervical glands. They are the most common incidental cervical finding — simple, anechoic, well-defined cysts within the cervical wall, sometimes multiple — and they require no treatment or follow-up. Do not confuse them with pathology.

Cervical polyps are echogenic focal masses that may protrude through the external os and cause contact bleeding; they are usually diagnosed clinically but can be seen in the canal. Cervical fibroids are leiomyomas arising in the cervical myometrium; a large cervical fibroid can distort the lower uterine segment and complicate both imaging and delivery.

Cervical Carcinoma

Cervical carcinoma (most often squamous cell carcinoma) is primarily a clinical and cytologic diagnosis made with Pap smear screening and biopsy — ultrasound plays a limited role. Exam points to remember:

• Early disease is usually sonographically occult — the cervix often looks normal
• Advanced disease appears as a bulky, irregular, hypoechoic cervical mass with increased vascularity
• An obstructing tumor can trap fluid above it, producing hydrometra or hematometra
• Staging and assessment of parametrial spread are done with MRI and clinical examination, not ultrasound

Cervical Length

Cervical length measurement is a routine part of obstetric imaging and uses the transvaginal approach with an empty bladder, measuring from the internal os to the external os along the canal. A normal mid-trimester cervix is roughly 25 mm or longer; a short cervix (<25 mm before about 24 weeks) is associated with an increased risk of preterm birth and may prompt cerclage or progesterone therapy. Funneling — beaking of the internal os with membranes protruding into the canal — further increases risk. In the nonpregnant patient, cervical length is not routinely measured, but an obviously shortened or absent cervix after surgery or with congenital anomaly should be documented.

Mullerian Duct Anomalies

Mullerian (paramesonephric) duct anomalies (MDAs) arise from the paired embryologic ducts that form the fallopian tubes, uterus, cervix, and upper vagina. Because the mesonephric (Wolffian) system drives renal development, uterine and renal anomalies travel together — about 30% of patients with an MDA have a renal anomaly, most commonly unilateral renal agenesis or an ectopic/pelvic kidney. Always evaluate the kidneys when you find a uterine anomaly.

Anomalies are grouped by the embryologic failure:

Septate vs. Bicornuate — the External Fundal Contour

The single most important imaging distinction is the shape of the outer serosal fundal surface, because it determines management. A septate uterus has a normal, convex, or only shallowly indented external contour with the division confined to the cavity; its fibromuscular septum has poor blood supply, so implantation on the septum leads to recurrent pregnancy loss, and treatment is hysteroscopic septum resection. A bicornuate uterus has a deep external fundal cleft (typically 1 cm or more) dividing the serosal surface; it more often causes malpresentation or preterm labor than early losses and is usually managed expectantly rather than resected.

3D ultrasound with reconstructed coronal imaging has become the sonographic gold standard for this evaluation: the coronal plane displays the external fundal contour and the cavity shape simultaneously, matching the information previously requiring MRI or combined laparoscopy/hysteroscopy. Imaging is best performed in the secretory phase, when the echogenic endometrium provides natural contrast against the myometrium.

Outflow Obstruction: Hematocolpos and Hematometra

An imperforate hymen (or a transverse vaginal septum) obstructs menstrual outflow at puberty. The classic patient is an adolescent with primary amenorrhea, cyclic monthly pelvic pain, and sometimes a bulging bluish membrane on examination. Retained menstrual blood distends the vagina (hematocolpos) and, as the obstruction backs up, the uterus (hematometra; together, hematometrocolpos). On ultrasound these appear as fluid-distended structures containing low-level internal echoes (old blood products), with the uterus sometimes displaced superiorly by the massively distended vagina. Recognizing this pattern in a teenager with cyclic pain and no menarche is a classic exam scenario.