Risk factors and imaging features of a rudimentary horn pregnancy: A case report and literature review.

A 33-year-old G5P0131 female at 15 weeks and 4 days gestational age by first trimester ultrasound presented with 1 week of lower pelvic and back pain and 1 day of severe right lower quadrant pain. She underwent a transabdominal ultrasound ( Fig. 1A and B ) that showed a pregnancy with cardiac activity in the left adnexa separate from a rightward deviated uterus. A pelvic MRI ( Fig. 1C ) and CT angiogram of the abdomen and pelvis were concerning for an abdominal pregnancy with a right superolateral placenta and blood supply from the left uterine and gonadal arteries. The patient had 3 reportedly unremarkable ultrasounds during this pregnancy and her course was uncomplicated prior to presentation. The patient’s obstetric history was notable for 2 spontaneous abortions at approximately 9 weeks gestational age and 1 biochemical pregnancy. She also had 1 emergent low transverse cesarean delivery at 32 weeks gestational age for a nonreassuring fetal heart tracing discovered during surveillance for fetal growth restriction. The cesarean delivery operative report noted a suspected 3 cm pedunculated fibroid on the left uterine cornu. The patient’s medical history was notable for a nonfunctional kidney, an inguinal hernia repair at 8 years old, and short stature (height 147 cm). Fig. 1 Transabdominal ultrasound (A, B) and pelvic MRI (C) both obtained at 15 weeks and 4 days gestational age. The final radiology ultrasound report noted a pregnancy (P) with cardiac activity in the region of the left adnexa, separate from the uterus (U) and not in continuity with the cervical canal (Black Arrows). Same day pelvic MRI demonstrated similar findings of a pregnancy (P) separate from a rightward deviated uterus (U), concerning for an extrauterine, abdominal pregnancy. Fig 1 dummy alt text Transabdominal ultrasound (A, B) and pelvic MRI (C) both obtained at 15 weeks and 4 days gestational age. The final radiology ultrasound report noted a pregnancy (P) with cardiac activity in the region of the left adnexa, separate from the uterus (U) and not in continuity with the cervical canal (Black Arrows). Same day pelvic MRI demonstrated similar findings of a pregnancy (P) separate from a rightward deviated uterus (U), concerning for an extrauterine, abdominal pregnancy. Given the concern for abdominal pregnancy with abnormal placentation and therefore the possibility of a highly morbid procedure, an exploratory laparotomy via a vertical midline skin incision was performed by the gynecologic oncology team. An 8-10 cm, well-circumscribed, intact mass immediately adjacent to the uterine fundus at the level of the left cornu was identified as the pregnancy and it appeared to be covered in uterine or tubal serosa ( Fig. 2A ). The mass was connected distally to a normal-appearing left fallopian tube and round ligament ( Fig. 2B ) and proximally to the main uterine body by a distinct stalk ( Fig. 2C ). The bilateral ovaries and the right fallopian tube appeared normal. A left salpingectomy and excision of the pregnancy mass was performed. The patient had an uncomplicated postoperative course. Fig. 2 (A, B) An 8-10 cm intact, unruptured pregnancy mass (P) delivered through a midline vertical abdominal incision with a normal left fallopian tube (White Arrows) and left round ligament (Black Arrows) attached. (C) A thick stalk secured by Heaney clamps (White Circle) connects the main uterine body (U) to the pregnancy mass (P). Fig 2 dummy alt text (A, B) An 8-10 cm intact, unruptured pregnancy mass (P) delivered through a midline vertical abdominal incision with a normal left fallopian tube (White Arrows) and left round ligament (Black Arrows) attached. (C) A thick stalk secured by Heaney clamps (White Circle) connects the main uterine body (U) to the pregnancy mass (P). Specimens reviewed by pathology included the excised pregnancy mass, the placenta, the left fallopian tube, and an intact male fetus. The pregnancy mass consisted of a cavity surrounded by a thin layer of myometrium with the focal presence of decidua, a clearly identified tubal insertion site not involved in placental implantation, and a thick stalk without a patent connecting channel to the main uterine cavity. Histologic sections showed loss of decidua throughout most of the placental implantation site with a noninvasive placenta, a diagnostic feature of placenta accreta spectrum Grade 1. Otherwise, the fallopian tube, placenta, and fetus were unremarkable and the placental villi and fetus corresponded to a gestational age of 16 weeks. These histologic findings confirmed the diagnosis of an RHP.

The patient described in the manuscript was provided a copy of the manuscript to review and signed an informed consent form permitting publication of the case report.

The rarity of RHPs and consequently low clinical suspicion and familiarity, coupled with the nonspecific clinical presentations and poor sensitivity of ultrasound imaging, create diagnostic challenges. However, there are certain risk factors associated with CUAs that should heighten the clinical suspicion for a rudimentary horn and prompt additional workup and monitoring, ideally prior to conception. Ultrasound is the initial imaging modality when an abnormal pregnancy is suspected, and there have been multiple proposed diagnostic criteria that appear to be both sensitive and specific for the diagnosis of an RHP. Additional imaging modalities such as MRI can improve diagnostic capabilities and help exclude other types of extrauterine pregnancies. A greater awareness of the risk factors and imaging findings associated with an RHP, as reviewed in this case report, may lead to a more accurate and earlier diagnosis, more appropriate and timely management, and ultimately a reduction in maternal and fetal morbidity and mortality.

The diagnosis of an RHP is challenging due to the nonspecific presenting symptoms, with the absence of symptoms in 8-14% of patients [ 2 , 6 ], and the poor sensitivity of ultrasound in the range of 26%-33% [ 1 , 7 , 8 ] that decreases as gestational age increases [ 9 ]. Other imaging modalities, including 3D ultrasound and MRI, may confer improved diagnostic capabilities, although sensitivity and specificity values have not been reported [ 8 , 10 ]. The presented patient underwent multiple ultrasounds in the current and past pregnancies that failed to identify a unicornuate uterus or rudimentary horn. The radiologic interpretation of the MRI and CT angiogram of the abdomen and pelvis upon presentation favored an abdominal pregnancy and an RHP was only diagnosed after pathologic examination of the specimen. However, a retrospective review of the patient’s medical record revealed several clinical features that should have raised suspicion for an RHP. Furthermore, review of the patient’s imaging studies revealed several findings proposed in prior literature to accurately diagnose an RHP and differentiate it from other extrauterine pregnancies. First, the patient exhibited a poor obstetric history, namely recurrent miscarriages and a prior preterm birth. The estimated prevalence of a CUA is 5.5% in the general population, but this risk increases to 8% in women with infertility, 13.3% in women with a history of recurrent miscarriage, and 24.5% in women with a history of both miscarriage and infertility [ 11 ]. Individuals with a unicornuate uterus have particularly poor pregnancy outcomes, with live birth rates of only 29.2% and a preterm delivery rate of 44% [ 2 ]. Second, the patient’s medical history is notable for a nonfunctional kidney, a repaired inguinal hernia, and short stature. CUAs are known to be associated with these anatomic abnormalities, more broadly renal, abdominal wall, and skeletal. Renal anomalies are especially common due to the closely related embryonic origins, with an estimated prevalence of 18.8% in those with any type of CUA and 29.5% in those with a unicornuate uterus [ 12 ]. One additional clue to this patient’s diagnosis is the suspected uterine fibroid at the left uterine cornu reported during her prior cesarean section, which instead was likely direct visualization of the rudimentary horn. Rodrigues et al. [ 9 ] presented a similar misdiagnosis, where an ultrasound prior to conception reported a 30 mm pedunculated fibroid later confirmed to be a rudimentary horn. Lastly, although not experienced by this patient, a history of primary dysmenorrhea, chronic pelvic pain, or endometriosis is present in 50-60% of patients with rudimentary horns due to hematometra formation and retrograde menstruation [ 7 , 10 ], and should therefore prompt consideration of a rudimentary horn if other risk factors exist. Several studies have reported imaging criteria used to diagnose an RHP. Mavrelos et al. [ 6 ] developed 2D ultrasound criteria to prospectively diagnose RHPs, which required all of the following criteria to be met: (1) a single interstitial portion of fallopian tube in the main uterine body, (2) a mobile gestational sac surrounded by myometrium clearly separate from the uterus, and (3) a vascular pedicle between the pregnancy and the unicornuate uterus, with the first criteria being the most critical in raising the suspicion for an RHP. Eight cases were identified using these criteria and confirmed with surgery and pathologic examination, with a sensitivity and specificity of 100%. Tsafrir et al. [ 4 ] proposed diagnostic criteria for first trimester unruptured RHPs based on 2 cases and a review of sonographic features described in the literature. The diagnosis requires (1) a pseudopattern of asymmetrical bicornuate uterus, (2) absent visual continuity between the cervical canal and the lumen of the pregnant horn, and (3) the presence of myometrial tissue surrounding the gestational sac. A supporting finding is placental hypervascularization suggestive of placenta accreta spectrum. After pathological confirmation of the RHP in this case, review of the prior imaging studies revealed several of these diagnostic criteria that were not commented on in the initial radiology reports. On both the ultrasound ( Fig. 3A ) and pelvic MRI ( Fig. 3B ) upon presentation, there is a thin layer of myometrial tissue surrounding the gestational sac. Characterization of the uterus on the pelvic MRI ( Fig. 3C ) shows a banana-shaped uterine cavity tapering at the apex to a single interstitial portion of fallopian tube consistent with a unicornuate uterus. On the CTA ( Fig. 3D ), there is a hypervascular uterus with a likely vascular connection to the rudimentary horn. These additional findings meet all of the criteria proposed by Mavrelos et al. [ 6 ], highlighting how improved familiarity with the imaging features may enhance provider diagnostic capabilities. Moreover, the patient’s initial dating scan also reveals a notably thin myometrium surrounding the gestational sac ( Fig. 4A ) and, in the only 2 available images of the uterus, there is a normal endometrial thickness but the absence of an intrauterine pregnancy and no demonstration of the uterine cornua ( Fig. 4 , Fig. 4 ). These findings should have prompted further workup to evaluate for a possible extrauterine pregnancy. Fig. 3 Post-surgery review of the imaging studies performed at 15 weeks 4 days gestational age. A transabdominal ultrasound (A) and pelvic MRI (B) demonstrate a pregnancy (P) in the left adnexa with a thin rim of myometrium, particularly anteriorly (White Arrows), and a posterolateral placenta (*). The pelvic MRI (C) also shows a banana-shaped uterine cavity (UC) tapering at the apex to a single interstitial portion of fallopian tube consistent with a unicornuate uterus. CT angiogram (D) of the abdomen and pelvis demonstrates a hypervascular uterus (U) with a likely vascular connection (Black Arrows) to the rudimentary horn pregnancy (P). Fig 3 dummy alt text Fig. 4 Post-surgery review of the nuchal translucency and initial dating ultrasound performed at 11 weeks and 3 days gestational age. (A) A thin rim of myometrium anterior to the pregnancy (P) is identified (Black Arrows). A thick rim of myometrium is not seen. The placenta is posterior right lateral (*). There is no documentation or imaging available of the cervix. Transabdominal characterization of the uterus is limited to 1 sagittal (B) and 1 transverse (C) view with a thickened endometrium (White Arrows) without visualization of an intrauterine pregnancy or uterine cornua. Fig 4 dummy alt text Post-surgery review of the imaging studies performed at 15 weeks 4 days gestational age. A transabdominal ultrasound (A) and pelvic MRI (B) demonstrate a pregnancy (P) in the left adnexa with a thin rim of myometrium, particularly anteriorly (White Arrows), and a posterolateral placenta (*). The pelvic MRI (C) also shows a banana-shaped uterine cavity (UC) tapering at the apex to a single interstitial portion of fallopian tube consistent with a unicornuate uterus. CT angiogram (D) of the abdomen and pelvis demonstrates a hypervascular uterus (U) with a likely vascular connection (Black Arrows) to the rudimentary horn pregnancy (P). Post-surgery review of the nuchal translucency and initial dating ultrasound performed at 11 weeks and 3 days gestational age. (A) A thin rim of myometrium anterior to the pregnancy (P) is identified (Black Arrows). A thick rim of myometrium is not seen. The placenta is posterior right lateral (*). There is no documentation or imaging available of the cervix. Transabdominal characterization of the uterus is limited to 1 sagittal (B) and 1 transverse (C) view with a thickened endometrium (White Arrows) without visualization of an intrauterine pregnancy or uterine cornua. Finally, several reports comment on clinical and sonographic findings that can be used to differentiate an RHP from other pregnancies with an apparent empty uterine cavity, as summarized in Table 1 . Specifically, tubal pregnancies, with an incidence of 1:80 to 1:100 live births, almost always present in the first trimester, lack myometrium surrounding the gestational sac, lack continuity between the gestational sac and uterine cavity, and have an empty uterus with 2 identifiable interstitial tubes [ 6 ]. Interstitial pregnancies, with an incidence of 1:2,500 to 1:5,000 live births, also typically present in the first trimester and have a gestational sac asymmetrically surrounded by myometrium and in continuity with the main uterine cavity, a finding known as the interstitial line sign [ 6 , 13 ]. An abdominal pregnancy, with an incidence of 1:10,000 to 1:30,000 live births [ 14 ], can result from primary implantation in the peritoneum, although most commonly arises secondary to rupture of a tubal ectopic pregnancy. Tubal rupture leads to implantation in the broad ligament resulting in a fixed, immobile mass, a gestational sac without surrounding myometrium, and a uterus with 2 identifiable interstitial tubes separate from the gestational sac [ 5 , 6 , 14 ]. A primary peritoneal abdominal pregnancy is less often on the differential for an RHP because it presents with the absence of adnexal abnormalities and a gestational sac surrounded by loops of bowel [ 15 ]. Lastly, abdominal pregnancies are commonly associated with fetal growth restriction and oligohydramnios due to abnormal placentation and rupture of the amniotic sac [ 5 , 6 , 16 ]. Table 1 Characteristics of different types of extrauterine pregnancies. Table 1 dummy alt text Trimester at presentation Ultrasound findings Associated abnormalities Continuity between gestational sac and uterine cavity Uterus with 2 identifiable interstitial tubes Myometrium surrounding gestational sac Vascular pedicle between gestational sac and uterus Mobile pregnancy separate from the uterus Fetal growth restriction and oligohydramnios Maternal skeletal, abdominal wall, and renal Rudimentary horn Varies No No Yes, symmetric Yes Yes No Yes Abdominal Varies No Yes No No Yes (peritoneal), No (adnexal) Yes No Interstitial First Yes Yes Yes, asymmetric No No No No Tubal First No Yes No No Yes No No Table adapted from Mavrelos et al. [ 6 ]. Characteristics of different types of extrauterine pregnancies. Table adapted from Mavrelos et al. [ 6 ]. Accurate and early diagnosis of an RHP is essential for surgical planning and minimizing adverse outcomes. Although the maternal mortality rate from an RHP has been reduced to 0.5% with advances in treatment, the morbidity is still high given that 80-90% of RHPs result in rupture by the second trimester, necessitating laparotomy, removal of the RHP and ipsilateral fallopian tube, and delivery of a previable fetus [ 5 , 6 ]. Although there have been cases of neonatal survival, the prognosis is poor due to the severity of premature delivery [ 2 , 7 , 9 ]. The recommended management of a rudimentary horn diagnosed prior to conception is laparoscopic excision to reduce the sequelae of retrograde menstruation and prevent an RHP [ 7 , 10 , 17 ]. First trimester RHPs have been successfully managed with primary laparoscopy, local injections of potassium chloride, lidocaine, and/or methotrexate, and even primary treatment with systemic methotrexate depending on the gestational age at diagnosis [ 4 , 6 , 9 , 18 ]. In all cases, definitive management with surgical excision of the rudimentary horn and ipsilateral fallopian tube is recommended given a high risk for a recurrent RHP [ 6 , 9 ]. In general, the surgical management of an RHP is technically straightforward regardless of whether a laparoscopic or open-abdominal approach is used [ 6 ]. On the other hand, the abnormal placentation with possible involvement of other organ systems in an abdominal pregnancy significantly increases maternal morbidity and mortality and necessitates laparotomy and the involvement of a multidisciplinary team including vascular and gastrointestinal surgeons [ 6 , 15 ]. In this case report, the gynecological oncology service performed the surgery given the initial concern for an abdominal pregnancy. This further emphasizes the importance of differentiating these 2 entities to determine the optimal treatment strategy and utilization of resources.

No generative AI and AI-assisted technologies were used in the preparation of this manuscript.

A rudimentary horn is a uterine cavity remnant formed by the incomplete development of a Müllerian duct, the embryologic precursor of the fallopian tubes, uterus, and upper vagina. It occurs in about 75%-84% of individuals with a unicornuate uterus [ 1 , 2 ], a type of congenital uterine anomaly (CUA) affecting approximately 1:1,000 women [ 3 ]. A rudimentary horn pregnancy (RHP) is rare, occurring in an estimated 1:76,000 to 1:150,000 pregnancies [ 2 ], and has a poor prognosis, with most cases resulting in second trimester rupture [ 2 , 4 , 5 ]. This case report details a second trimester unruptured RHP initially mischaracterized as an abdominal pregnancy. Although the patient had multiple risk factors for a CUA and imaging findings suggestive of an RHP, the diagnosis was only confirmed on pathologic examination after surgical excision. To enhance provider familiarity and improve diagnostic accuracy of this uncommon condition, we also review the key clinical and imaging features of an RHP.