Incarcerated gravid uterus is a rare obstetric condition wherein the gravid uterus, becomes trapped in the pelvis between the sacral promontory and pubic symphysis. The estimated incidence ranges between 1 in 3,000 to 1 in 10,000 pregnancies1. Known risk factors are uterine retroversion, large uterine fibroids, endometriosis, pelvic adhesions, uterine anomalies, and history of prior abdominal or pelvic surgery. Although majority of retroverted uteri correct their position spontaneously as pregnancy advances, incarceration may occur, leading to complications such as miscarriage, preterm labor and urinary retention. This case indicates a poor pregnancy outcome due to incarcerated uterus and highlights complexicity and limitations of diagnostic imaging due to anatomical distortion caused by a large fibroid.

CASE DESCRIPTION A 37-year-old primigravida, around 13 weeks pregnant was referred to the Fetal Medicine (FM) unit for ultrasonography (USG) with history of heavy bleeding per vaginum. This was a spontaneous conception. Her NT scan done outside reported large posterior wall subserosal fibroid of 13.5 x 9 cm in size. The rest of the scan was reported to be normal. Her Combined screening test showed high risk for Trisomy 13 and Trisomy 18 (risk >1:5), intermediate risk for Trisomy 21 (1:691). She presented in the emergency department (ER) with active bleeding. On clinical examination in the ER per abdomen uterus felt like that of 18-20 weeks’ size pregnancy, per speculum examination revealed clots in the vagina, cervix was not visualised. Per vaginal examination revealed a large posterior fibroid of about 10 cm, cervix was anteriorly displaced with open internal os indicating an inevitable miscarriage. She was admitted overnight for observation. The patient gave history of passing a large clot overnight followed by cessation of bleeding. Transabdominal and transvaginal scan of the pelvis was done the next day of her admission. Ultrasound findings showed enlarged retroverted uterus, empty uterine cavity and no fetus or products of conception were seen in the uterine cavity (Fig 1and Fig 2). Given the history of settling down of per vaginal bleeding following passage of a large clot and subsequent USG, findings were suggestive of a complete miscarriage. Following this, she was discharged home with advice of repeating the scan. Patient reported back to the ER almost 3 weeks after the first admission with complaints of spontaneous expulsion of fetus at home. She underwent curettage for retained products of conception thereafter. A pelvic USG in non-pregnant state done later revealed a 118 mm X 69 mm X 86 mm (370 cc) large subserous uterine fibroid compressing the cervix. A few 5 mm, 9 mm, 20 mm diameter small fibroids were also seen. Figure 1: Transabdominal USG of patient with Incarcerated uterus Figure 2: Transvaginal USG of patient with Incarcerated uterus

This case represents an incarcerated gravid uterus. A gravid uterus is said to be ‘incarcerated ‘when it is trapped between the sacral promontory and pubic symphysis during pregnancy 5. This is a rare condition with reported incidence of around 1:3000 to 1:10,000 pregnancies. A retroverted uterus in the first trimester, endometriosis, leiomyomas , deep sacral concavity with an overlying sacral promontory, previous abdominal or pelvic surgery, pelvic or uterine adhesions, ovarian cysts, multifetal gestation, uterine anomalies, uterine prolapse, and uterine incarceration in a prior pregnancy are known risk factors for incarcerated gravid uterus 1,2,4. Normally a retro positioned gravid uterus corrects itself spontaneously as the gestation advances with fundus of the uterus rising above the sacral promontory. In incarcerated uterus, the uterus gets trapped under the sacral promontory and the cervix gets displaced anteriorly behind the symphysis pubis 10. In our patient, clinical examination did reveal an anteriorly displaced internal os. This case also shows us how a large uterine fibroid can misguide us and an intrauterine pregnancy can be missed on ultrasound scan especially in the first trimester. An accurate and detailed ultrasound depends on multiple technical and biological factors and is known to have limitations. Factors such as penetration of sound waves, availability of acoustic window can contribute to its ability or inability to diagnose or identify pathology. For an image of an organ to be seen on an ultrasound there must be two structures of different acoustic properties. In cases where this kind of window is not available the definition of detailed anatomy becomes difficult. Large uterine fibroid that fills the entire pelvic cavity is known to distort the anatomy of the uterus as well as other pelvic organs. In a gravid uterus, the gestational sac can get compressed and the contents of the sac can get obscured, in presence of a large fibroid. Presence of additional fibroids complicate the situation further since each fibroid can have a different echo texture 3. In the present case, the echotexture of the large fibroid, its posterior location and distorted echotexture of the myometrium and patient’s history of cessation of bleeding after passage of a large clot appear to be the major contributors to the conclusion of a complete miscarriage in the ultrasound report. In incarcerated gravid uterus, an anteriorly displaced cervix is also difficult to locate on sonography 10. This condition has its own list of complications such as spontaneous miscarriage, urinary retention, preterm labour, uterine dystocia 4. Approximately 10% to 30% of women with uterine fibroids develop complications during early and late pregnancy 6. Large fibroid uterus is known to cause poor implantation of pregnancy, placental abruption leading to miscarriage / pregnancy loss 7,8. Spontaneous miscarriage rates are greatly increased in pregnant women with fibroids compared with control subjects without fibroids (14% vs 7.6%, respectively) 6,10. Possible known causes are increased uterine irritability and contractility, the compressive effect of fibroids and compromise to the blood supply of the developing placenta and fetus 6. Patient’s blood test report (combined screening test report) that showed high risk for Trisomy 13 and Trisomy 18 WAS also indicative of placental dysfunction. The overall prognosis and outcome of such pregnancy is known to be poor 9.

Incarcerated gravid uterus should be considered when early pregnancy presents with distorted pelvic anatomy, especially in presence of large fibroids and retroverted uterus. Ultrasound imaging in such cases can be limited and misinterpreted. Clinical correlation, awareness of risk factors, and high index of suspicion are essential to avoid misdiagnosis and guide appropriate management. Clinical Significance This case highlights the importance of recognizing the diagnostic limitations of ultrasound in early pregnancy with distorted anatomy. Awareness of rare conditions like incarcerated gravid uterus and understanding the impact of large fibroids on placental function and clinical outcomes can help clinicians make informed decisions, prevent complications, and improve patient care. List of Abbreviations • FM – Fetal Medicine • NT – Nuchal Translucency • ER – Emergency Room • USG – Ultrasonography • mm – millimeters • cc – cubic centimeters

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