Ultrasound diagnosis of an unruptured rudimentary horn pregnancy with placenta accreta: a case report.

A 26-year-old primigravida at 20 weeks of gestation presented for a routine prenatal checkup without reporting any physical discomfort. Previous assessments had confirmed normal fetal development. The transabdominal ultrasound examination revealed an echogenic lesion in the left uterine corpus measuring 8.4 × 6.5 × 6.6 cm, with a homogeneous myometrium and an endometrial thickness of 1.5 cm, and adjacent to the right uterine wall, a gestational sac measuring 13.6 × 10.4 cm was detected ( Figure 1 ). This sac was not connected to the left uterine cavity and was approximately 1.6 cm away from it ( Figure 2 ). Fetal measurements revealed a biparietal diameter of 4.9 cm, head circumference of 17.5 cm, abdominal circumference of 15.5 cm, and femur length of 3.0 cm, with a fetal heart rate of 161 beats per minute. The fetal spine was aligned and continuous. Although the placenta was visible, it was poorly demarcated from the myometrium ( Figure 3 ). Fetal heart activity and movement were observed, and the amniotic fluid depth was 5.7 cm. The ultrasound report indicated: ① A pregnancy adjacent to the right uterine wall, corresponding to 19 weeks and 6 days of gestation,the possibility of a rudimentary horn pregnancy or other types of ectopic pregnancies cannot be ruled out;② The potential for placenta accreta was also considered. (1) The normal uterus on the left side of the patient is shown on the left side of the picture. (2) The right image shows both uteri visible simultaneously. Distance between right residual uterine pregnancy and left non-pregnant uterine cavity (R represents the right residual horn uterus with pregnancy, L represents the left normal uterine body). (A) This is an image of the placenta in our patient, with a blurred demarcation between the placenta and the myometrial wall shown by yellow arrows. (B) This picture shows a clear boundary between the placenta and the myometrial wall, as indicated by the double yellow arrows. A few days after the ultrasound examination, the patient experienced severe abdominal pain, primarily localized to the lower abdomen, which was intolerable and accompanied by uterine contractions. Given the potential for serious complications, the clinical team conducted a thorough assessment of the patient's condition and recommended an emergency cesarean section. After careful consideration, the patient consented to the surgical intervention. Intraoperative findings revealed a rudimentary uterus located on the right side, measuring approximately 9 × 7 cm. During manipulation of the right uterine corpus, spontaneous rupture of the myometrium occurred, leading to the release of clear amniotic fluid and the delivery of a stillborn male infant. The placenta was fully implanted within the rudimentary uterus. Surgical resection of the right rudimentary uterus and fallopian tube was performed, while the left fallopian tube and both ovaries were preserved. The patient's postoperative recovery was uneventful, and she was discharged on the fifth postoperative day. Histopathological examination confirmed the diagnosis of a rudimentary horn pregnancy.

A rudimentary uterus is a congenital uterine malformation resulting from the incomplete fusion of the paramesonephric ducts during embryonic development. This anomaly typically leads to the formation of a functional unicornuate uterus on one side, while development on the opposite side is arrested, leading to the absence of lower ductal segments and resulting in a rudimentary uterus without a cervical structure. Current literature indicates a concerningly low rate of correct preoperative diagnosis, reported between 22% and 29%, with diagnostic sensitivity further declining as gestation progresses ( 1 ). In addition, rudimentary uterus can lead to several complications, such as hematochezia, dysmenorrhea, endometriosis, miscarriage, preterm labor, and ectopic pregnancy ( 2 , 3 ). It has been found that uterine anomalies affect approximately 5% of women, but pregnancies within a rudimentary uterus are exceptionally rare, with an estimated incidence ranging from 1/160,000 to1/76,000 ( 4 ) cases and carry a high risk due to the presence of myometrial dysplasia, which increases the likelihood of rupture. In the event of rupture, severe intra-abdominal hemorrhage may occur and may be life-threatening to the mother. Thus, early diagnosis and intervention are essential to prevent these catastrophic outcomes.

Female genital anomalies are a group of congenital disorders caused by abnormal development of the paramesonephric ducts (i.e., Müllerian ducts) during the embryonic period. They include developmental abnormalities of the uterus, cervix, and vagina ( 5 ), and are often accompanied by anomalies in other systems, most commonly the kidneys and the skeletal system ( 6 ). Currently, various studies report that the prevalence of female reproductive organ malformations in the general population ranges from 0.5% to 6.7% ( 7 ). These patients typically have higher rates of miscarriage (25%), preterm birth (15%–25%), or cervical insufficiency (38%) ( 8 ). Other adverse outcomes include fetal growth restriction, abnormal fetal presentation, placental abnormalities, and ectopic pregnancy ( 9 ). In a study of more than 300 uterine pregnancies associated with Müllerian duct anomalies, patients with a unicornuate uterus had the highest rates of uterine rupture (4.7%) and in vitro fertilization-embryo transfer (12.6%) ( 10 ). The unicornuate uterus, often accompanied by a rudimentary horn, results from incomplete development of one paramesonephric duct. It occurs in approximately 1 in 40,000 to 1 in 10,000 cases and is frequently associated with infertility and recurrent pregnancy loss, as the rudimentary horn presents an obstructive uterovaginal malformation ( 11 ). At the end of 2021, the ARSM Task Force created the 2021 ARSM Mueller Tube Development Abnormal Classification System (ARSM MAC 2021), they classified the unicorned uterus into five categories, including: (1) R/L unicornuate uterus, (2) R/L unicornuate with R/L distal atrophic uterine remnant, (3) R/L unicornuate with R/L distal uterine remnant with functional endometrium, (4) R/L unicornuate with R/L associated atrophic uterine remnant, (5) R/L unicornuate with R/L uterine horn communicating at level of cervix ( Figure 4 ) ( 12 ). Classification of unicornuate uterus [American society for reproductive medicine (ASRM) müllerian anomalies classification 2021]. During the ultrasound examination, we observed that the gestational sac containing the fetus in the rudimentary horn appeared to protrude outward at an angle from one side of the normal uterus. It lacked the typical morphology of the normal uterus, had a thinner myometrium, and was spatially separated from the normal uterine cavity. Although this presentation may resemble other types of ectopic pregnancies, fetuses in other ectopic pregnancies generally do not grow significantly because they often rupture early. It has been proposed that certain ultrasound findings ( 13 ), including a very thin perimyometrial layer, placental implantation, an empty uterine cavity, and a separate adnexal pregnancy sac, may aid in the diagnosis of rudimentary horn pregnancy. Our case exhibited several of these features. Unlike pregnancies in a bicornuate uterus, which are typically characterized by endometrial continuity between the pregnancy sac and the contralateral uterine horn, our case demonstrated discontinuity between the pregnancy sac and the normal uterine cavity. Previous reports have indicated that 80%–90% of rudimentary horn pregnancies (RHP) result in horn rupture during the second trimester ( 14 ). Only 14% of cases are diagnosed before clinical symptoms appear. Most cases of RHP are challenging to diagnose and are often identified after rupture, which can lead to emergency surgery, blood transfusions, and increased morbidity. In our case, the patient had not been previously examined in our hospital, which caused great difficulty in our diagnosis, but due to the abnormal position of her gestational sac and the presence of a unicornuate uterus on the left side, our doctors suspected the possibility of a stumpy uterine pregnancy, but could not completely rule out other ectopic pregnancies, and most of the previously reported cases in the literature were detected in the early stages of pregnancy, with very few being detected in mid- to late-stage. And it is easy to misdiagnose as normal intrauterine pregnancy. In the majority of case reports, patients either presented with clinical symptoms or had a known history of uterine anomalies ( 14 ). After a comprehensive review of the literature, we found that there is currently no unified standard for the extent to which ultrasound can diagnose the deformity of a unicornuate uterus. According to our clinical experience, with the more comprehensive knowledge of uterine malformations by current physicians and the development of ultrasound technology, unicornuate uterine malformations are generally easier to diagnose before pregnancy, and are not easily distinguishable when the morphology of the uterine cavity changes after pregnancy. It has been reported that only 22% of gynecologic cases and 29% of obstetric cases can be accurately diagnosed before surgery, indicating that the majority of cases can only be clearly diagnosed after surgery ( 15 ). Pregnancy within a rudimentary horn is a rare form of ectopic pregnancy. It is caused by insufficient development of the myometrium, thinning of the decidual layer, and abnormal invasion of chorionic villi into the submembranous membrane, which leads to placental implantation. These conditions predispose the pregnancy to hemorrhage and uterine rupture, particularly in the second trimester. In rare cases, however, such pregnancies may continue to term. The risk of rupture in a non-communicating rudimentary horn depends on the degree of muscular development and hypertrophy, with rupture occurring between 5 and 35 weeks of gestation ( 16 ). The potential for rupture increases significantly after 16–20 weeks of gestation, presenting with symptoms similar to those of tubal pregnancy, such as abdominal pain and bleeding. Park et al. ( 17 ) reported that the risk of rupture can reach up to 50% in the second trimester. A thicker myometrium of the rudimentary horn compared to that of the fallopian tube may delay rupture, but when it occurs, the outcome can be life-threatening. Immediate surgical intervention is essential to prevent severe internal hemorrhage. In this regard, Jayasinghe et al. ( 18 ) recommend preemptive surgical removal of a rudimentary horn to prevent complications, such as dysmenorrhea and endometriosis, resulting from retrograde menstruation. This intervention is particularly important for patients with symptomatic non-communicating rudimentary horns, even before pregnancy occurs. In reports on mid- and late-term pregnancies in women with uterine malformations, most cases required emergency surgical intervention based on the severity of the condition ( 19 ); however, other similar reports describe successful conservative management ( 20 ) , with these cases primarily involving patients in the early stages of pregnancy. Preoperative rupture is one of the most serious complications of pregnancy in a rudimentary horn of the uterus, potentially leading to shock and life-threatening conditions ( 21 ). If a diagnosis of rudimentary horn pregnancy (RHP) is confirmed, surgical resection of the rudimentary horn is recommended due to the high risk of rupture during the second trimester ( 14 ). For patients with stable conditions or asymptomatic patients who refuse surgery, conservative treatment with methotrexate (MTX) may be considered, or surgical intervention may be performed based on the clinical presentation. Surgery serves both diagnostic and therapeutic purposes. Surgical options include radical resection of the rudimentary horn and the ipsilateral fallopian tube, or uterine-preserving evacuation of the gestational tissue. For non-communicating cases, the combination of MTX and laparoscopic surgery can reduce intraoperative blood loss ( 22 ). If abnormal uterine development is detected during pregnancy, the specific type of malformation should be promptly identified, and its impact on both maternal and fetal outcomes should be assessed, with appropriate increases in the frequency of prenatal monitoring. In the event of premature rupture of membranes or signs of preterm labor, antibiotics and tocolytics should be judiciously administered under close surveillance to prolong gestation as much as possible ( 23 ). The timing and mode of delivery should be individualized to minimize complications for both mother and neonate. Although Müllerian duct anomalies are not an absolute indication for cesarean delivery, operative delivery should be considered in cases of malpresentation, fetal growth restriction (FGR), or a history of adverse pregnancy outcomes. Additionally, close attention must be paid to postpartum complications, including retained placenta and postpartum hemorrhage, to ensure maternal and neonatal safety ( 10 ). Rudimentary horn pregnancy is an exceedingly rare condition that often presents without early, distinctive symptoms, making it highly susceptible to oversight and misdiagnosis. Consequently, the diagnosis of this condition predominantly relies on imaging diagnostics. Siwatch et al. ( 24 ) considered ultrasound as the most crucial diagnostic tool for identifying rudimentary horn pregnancies and emphasized the need for detailed ultrasound scanning to assess uterine anatomy and accurately locate the pregnancy, facilitating timely diagnosis and management. A report by Julio Elito Júnior et al. ( 25 ) described a heterotopic twin pregnancy, with one fetus developing within the cavity of a unicornuate uterus and the other in a non-communicating rudimentary horn. In that case, MRI was performed to confirm the diagnosis, while ultrasound was employed to monitor myometrial thickness and guide delivery planning, resulting in both fetuses surviving. The morphological changes of the uterus during pregnancy pose significant challenges for the diagnosis of uterine anomalies. As gestational age increases, the field of view in ultrasound examinations can be easily affected; therefore, women planning pregnancy should undergo relevant examinations beforehand to promptly detect uterine anomalies that may threaten maternal safety during pregnancy. For patients in early pregnancy, when an intrauterine gestational sac is not identified and a suspected adnexal mass is detected adjacent to the uterus, special attention should be paid to observing whether the uterine cavity morphology is abnormal, the relative position of the gestational sac to the uterus and endometrial cavity, and whether the gestational sac communicates with the cervical canal. When two-dimensional ultrasound is insufficient for diagnosis, three-dimensional ultrasound or MRI may be considered for further evaluation. These implications can directly inform clinical practice and guide the development of more effective treatment strategies.