An incarcerated uterus is a rare but serious complication of pregnancy. This condition occurs when the growing uterus becomes physically trapped within the bony confines of the pelvis, preventing its normal ascent into the abdominal cavity. Affecting approximately one in 3,000 to 10,000 pregnancies, prompt identification and management are important for maternal and fetal well-being. This complication typically manifests during the second trimester of gestation, often presenting with symptoms initially mistaken for common pregnancy discomforts.
Defining Uterine Incarceration
Uterine incarceration occurs when the growing uterus becomes wedged and fixed within the pelvic cavity. The entrapment happens between the sacral promontory (at the top of the sacrum) and the pubic symphysis (at the front of the pelvis). This mechanical impaction prevents the uterus from rising above the pelvic brim, which normally occurs around the 12th to 16th week of pregnancy.
The underlying anatomical issue is almost always a retroverted uterus, where the uterus naturally tips backward toward the spine. While up to 15% of first-trimester pregnancies involve a retroverted uterus, the pressure of the growing fetus usually causes it to spontaneously move forward. If this correction fails, the fundus (top of the uterus) becomes trapped beneath the sacral promontory.
As the fetus and uterus enlarge, the body of the uterus remains fixed in the sacrum’s hollow. The cervix is simultaneously pulled upward and pressed against the pubic bone. This distortion usually becomes clinically apparent in the early second trimester, typically between the 12th and 20th weeks of gestation.
Identifying the Causes and Risk Factors
The primary cause of uterine incarceration is the persistent retroversion of the uterus that fails to correct itself as the pregnancy progresses. Several factors can interfere with the natural process of the uterus moving out of the pelvis and into the abdomen, essentially fixing the uterus in its backward-tilted position.
Pelvic adhesions are a significant risk factor, often resulting from previous abdominal or pelvic surgeries, endometriosis, or pelvic inflammatory disease. These bands of scar tissue tether the uterus to surrounding structures, physically impeding its ability to change position. Uterine abnormalities, such as leiomyomas (fibroids), especially those on the posterior wall, can also block the normal ascent.
Other predisposing factors include a deep sacral concavity or a contracted pelvis, which limits the available space for expansion. A prior history of uterine incarceration in an earlier pregnancy also increases the risk of recurrence. These conditions prevent the necessary anatomical shift, leading to the uterus becoming wedged in the pelvic bowl as the fetus grows.
Recognizable Symptoms and Clinical Presentation
The symptoms accompanying uterine incarceration arise from the mechanical compression of pelvic structures by the trapped, enlarging uterus. The most common and often first noticeable symptom is acute urinary retention, the inability to empty the bladder. This occurs because the upward and forward displacement of the cervix and lower uterine segment compresses the urethra and bladder neck, obstructing urine flow.
Patients may also experience urinary frequency, difficulty passing urine (dysuria), or paradoxical incontinence (involuntary leakage from an overly full bladder). Pressure on the rectum can lead to bowel symptoms such as constipation, rectal pressure, or tenesmus.
Pain is a frequent complaint, manifesting as lower abdominal, pelvic, or back discomfort. A less obvious but important clinical sign is that the expected abdominal growth, the “baby bump,” may be absent or less prominent than expected for the gestational age. This happens because the uterus is expanding backward within the pelvis rather than forward into the abdomen.
Diagnosis and Treatment Options
A presumptive diagnosis of uterine incarceration often begins with a physical examination, particularly when a patient in the second trimester presents with classic urinary symptoms. The examiner may note that the uterine fundus cannot be felt above the pubic bone in the abdomen, despite the advanced gestational age. A pelvic examination will typically reveal that the cervix is displaced extremely high and forward, positioned against or even above the pubic symphysis, making it difficult to visualize or palpate.
Imaging studies are necessary to confirm the diagnosis and assess the extent of the entrapment. Ultrasound, both transabdominal and transvaginal, is the primary diagnostic tool. It can visualize the retroverted fundus deep in the sacral hollow and the elongated, anteriorly displaced cervix. Pelvic Magnetic Resonance Imaging (MRI) is sometimes used for more complex cases or when the diagnosis remains uncertain, as it provides clear anatomical detail of the trapped uterus, the cervix’s position, and any associated pelvic masses.
Treatment strategies are tiered, beginning with conservative measures. Initial management involves draining the bladder with a urinary catheter, which can sometimes relieve enough pressure to allow for spontaneous repositioning. Positional changes, such as placing the patient in the knee-chest position or on all fours, may be attempted to use gravity to encourage the uterus to move out of the pelvis.
If conservative methods fail, the next step is manual reduction, a technique where a clinician applies gentle, sustained upward pressure through the vagina to the trapped uterine fundus. This procedure is often performed with the patient under regional or general anesthesia to ensure muscle relaxation and minimize discomfort. Following a successful reduction, a pessary may be inserted to maintain the uterus in the correct position for the remainder of the pregnancy.
If manual reduction is unsuccessful, or if the diagnosis is made later in gestation (after 20 weeks), more invasive procedures may be considered. These include colonoscopic reduction, which uses a scope to apply pressure from the rectum, or surgical intervention via laparoscopy or laparotomy. Once the uterus is successfully repositioned, the pregnancy can typically continue with close monitoring. Untreated incarceration carries risks such as uterine rupture, miscarriage, or premature delivery.