An incarcerated uterus is a rare complication of pregnancy that occurs when the enlarging uterus becomes wedged within the pelvic cavity, preventing its normal ascent into the abdomen. This condition, estimated to affect approximately one in every 3,000 pregnancies, typically develops in the late first or early second trimester, around 12 to 16 weeks of gestation. The entrapment happens when the growing uterus is caught below the bony structures of the pelvis. This mechanical obstruction can lead to significant problems for both the mother and the developing fetus if not promptly identified and corrected.
The Mechanism of Uterine Incarceration
The process begins with a uterus that is retroverted or retroflexed, meaning it is tilted backward toward the spine. This anatomical variation is common, occurring in up to 20% of women in the first trimester, and usually corrects itself spontaneously. Ordinarily, the expanding uterine fundus rises out of the pelvis and rotates forward into the abdominal cavity by the end of the first trimester.
In an incarcerated uterus, this natural repositioning fails, and the growing fundus becomes physically trapped in the hollow of the sacrum. It gets wedged between the sacral promontory and the symphysis pubis. This entrapment can be promoted by pre-existing conditions that limit uterine mobility, such as adhesions from prior pelvic surgery, endometriosis, or the presence of uterine fibroids.
As the pregnancy continues to grow within the confined pelvic space, the uterus becomes fixed, and its shape distorts. The cervix is pulled upward and forward, becoming elongated and displaced against the pubic bone. Meanwhile, the lower uterine segment may thin out and protrude into the abdominal space, a process known as sacculation, while the fundus remains deep within the pelvis.
Recognizing the Symptoms and Complications
The clinical presentation results from the physical compression of surrounding pelvic organs by the trapped, enlarging uterine mass. The most common symptom is acute or chronic urinary retention, which is difficulty or inability to empty the bladder. This occurs because the upwardly displaced cervix presses directly against the urethra and the neck of the bladder, mechanically obstructing the flow of urine.
Patients may also report other urinary and rectal symptoms:
- Increased frequency, urgency, or painful urination (dysuria)
- Overflow incontinence
- Constipation, rectal pressure, or tenesmus
- Lower abdominal, pelvic, or back pain due to mechanical stretching and pressure on surrounding nerves
If the condition goes unrecognized and uncorrected, it can lead to severe maternal and fetal complications. Prolonged urinary retention can cause urinary tract infections and potentially kidney damage. Risks to the pregnancy include miscarriage, fetal growth restriction, preterm labor, and, in rare cases, uterine rupture.
Diagnostic Procedures
Identifying an incarcerated uterus relies on patient history, physical examination, and medical imaging. During a pelvic examination, the cervix is often displaced high up and forward, making it difficult to visualize or reach. The practitioner may also feel the soft, enlarged mass of the uterine fundus fixed deep within the posterior cul-de-sac of the pelvis, behind the cervix.
Imaging is necessary to confirm the diagnosis and visualize the abnormal anatomy. Ultrasound, utilizing both transabdominal and transvaginal approaches, is typically the initial tool used. Sonographic findings often include a retroverted uterus with the fetal head or body positioned deep within the pelvis, and a bladder that appears elongated and displaced superiorly.
Magnetic Resonance Imaging (MRI) is often considered superior to ultrasound, especially in later stages, as it provides clearer detail of the distorted maternal anatomy. MRI can accurately delineate the relationship between the trapped uterine fundus, the cervix, the bladder, and the bony pelvis. The combination of a high-riding cervix and the uterine fundus trapped posteriorly is generally conclusive for diagnosis.
Treatment Options and Prognosis
Management focuses on releasing the trapped fundus and restoring the uterus to its normal, forward-tilted position. For early cases, initial management involves decompressing the bladder, typically by inserting a Foley catheter, to provide immediate relief and potentially allow for spontaneous repositioning. The patient may also be instructed to adopt positions that encourage the uterus to shift, such as the knee-chest position, several times a day.
If conservative measures are unsuccessful, manual repositioning is often the next step. This procedure involves a healthcare professional applying gentle, continuous pressure through the vagina to lift the trapped fundus out of the pelvic hollow. It is often performed under regional or general anesthesia to relax the muscles, increasing the chance of success.
When manual techniques fail, or if the diagnosis is made later in the second trimester, surgical intervention may be necessary. Minimally invasive laparoscopy can be used to free the uterus from adhesions and guide its repositioning. If the condition persists until near term, a laparotomy (open abdominal surgery) may be required.
The prognosis is generally favorable when the uterus is successfully repositioned. Once released, the uterus typically ascends into the abdomen, allowing the pregnancy to progress normally, often resulting in a full-term vaginal delivery. Delayed diagnosis or failed repositioning carries a risk of needing a cesarean delivery, as the distorted anatomy can prevent vaginal birth.