The term incarcerated gravid uterus refers to a pregnant uterus that is entrapped in the pelvis between the sacral promontory and pubic symphysis (figure 1A-B). It has been reported to occur in 1 in 3000 to 10,000 pregnancies .
In very early pregnancy, the uterus is retroverted (retroflexed) in up to 20 percent of women . As the uterus enlarges during the first trimester, the fundus normally rises from the hollow of the sacrum to an anterior ventral position, spontaneously correcting any retroversion. In rare cases, however, the fundus becomes wedged below the sacral promontory, where it continues to enlarge . Concomitantly, the cervix becomes displaced cephalad against or above the symphysis pubis and pushes against the urethra and bladder, which interferes with normal voiding.
Eventually, the posterior pelvis becomes too small to accommodate the increasing size of the fundus; this typically occurs when the pregnancy progresses beyond 20 weeks of gestation. At this point, the anterior lower uterine wall begins to thin and balloon into the upper abdomen, developing a sacculation . This is termed ‘inverted polarity’ because the anterior lower uterine wall becomes cephalad to the fundus, which is posterior-caudal. Concomitantly, both the bladder and the cervix are pulled up into the abdominal cavity, towards the umbilicus. The cervix can stretch to 10 cm or more in length, such that the internal os becomes located above the symphysis pubis, and occasionally above the bladder .
Conditions that may inhibit the fundus of the enlarging uterus from ascending out of the sacral hollow include: adhesions related to previous pelvic surgery, pelvic inflammatory disease, or endometriosis; large fibroids; uterine malformation; or a deep sacral concavity with an overhanging promontory and/or laxity of the supporting tissues [4,6]. Incarceration can also occur in the absence of predisposing factors, but this is uncommon [7,8].
Patient presentation — Typically, the patient presents at 14 to 16 weeks of gestation with symptoms, often vague, related to pressure on the anatomic structures adjacent to the entrapped enlarging uterus. The most common symptoms are pain and progressive difficulty voiding. The pain may be abdominal, suprapubic, or in the back; or may be limited to pelvic discomfort or a feeling of pelvic fullness. Urinary symptoms include frequency, dysuria, sensation of incomplete emptying, dribbling small volumes due to overflow incontinence, and, often, urinary retention. Gastrointestinal symptoms such as rectal pressure, tenesmus, and worsening constipation may occur due to compression of the rectum [4,6]. Vaginal bleeding has also been described. Symptoms can be intermittent, resolving for a period of time and then returning weeks later.