Ultrasound of the Month: Lower Uterine Segment Gestation

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The Case…

The patient is a young G2P1 female with a medical history notable for a prior miscarriage who presents to the Emergency Department (ED) with complaints of abdominal pain and vaginal bleeding. The patient reports cramping abdominal pain that was present on the morning of presentation. She stood up from bed and had a large amount of bright red blood from her vagina. She estimates that she is about 6 weeks pregnant but notes her last menstrual period was 4 months prior to presentation. She has had no imaging or prenatal care.

Her temperature is 36.7 C, HR 52 bpm, BP 146/97 mmHg, respiratory rate 18 per minute, and pulse oximetry 100% on room air. Her exam is significant for mild suprapubic abdominal tenderness with no is peritoneal signs. On vaginal exam, she has a large clot within the vaginal vault, as well as an open os on palpation of the cervix. There is no significant adnexal tenderness or cervical motion tenderness. 

AND NOW FOR THE ULTRASOUND IMAGES…

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What do you see on ultrasound?

These four images, the first two transabdominal and second set transvaginal, are consistent with an irregularly shaped gestational sac in the lower portion of the uterus. No fetal movement or cardiac activity is seen.

Ultrasound Pearls

When a patient arrives in the ED with abdominal pain in the setting of a positive pregnancy test, the typical differential expands to include ectopic pregnancy. Ultrasound is the initial imaging modality of choice used to determine the location of a potential pregnancy.

Typically a transabdominal ultrasound is performed initially to determine if an intrauterine pregnancy (IUP) is able to be identified. The probe, curvilinear or phased array, is placed just superior to the pubic symphysis with the indicator towards the patient’s head. The probe is then fanned along the sagittal plane to capture the uterus. Once identified, clips of the full sweep are taken in both the longitudinal and transverse planes. The adnexa can be identified by sweeping the probe slightly to the left or right and fanning through the ovaries as described above. If an IUP is not identified, a transvaginal exam should be performed. The patient’s bladder should be emptied and the patient placed in a lithotomy position to optimize image acquisition. The probe is inserted into the vagina with a sagittal orientation. Similar to the transabdominal approach, the uterus and both adnexa are evaluated in both longitudinal and transverse views with close attention paid to the location of the pregnancy.

The first sonographic finding of pregnancy is the gestational sac, typically visible at around 4-5 weeks gestation. This is seen as a round or oval anechoic structure within the endometrium surrounded by a thin regular echogenic wall. While this is the first finding seen in a normal IUP, a pseudo-gestational sac can also be seen in the setting of ectopic pregnancy. Around the 5th week, a yolk sac will appear inside the gestational sac as a round echogenic ring with an anechoic center. Once the yolk sac or an embryo with a heartbeat is found, a definitive IUP is confirmed.

A normal IUP is located in a paracentric location within the decidua 1. A gestational sac seen in the lower portion of the uterus, close to the cervix, is considered to be located in the lower uterine segment. The lower uterine segment by definition does not develop until later in pregnancy as the uterus expands. However, this term can be used in early pregnancy to include the tissue superior to the external cervical os and inferior to the portion of the uterus where the peritoneum of the vesicular pouch connects with the uterus 2. When the gestational sac and yolk sac are seen in the lower uterine segment, there is concern for cervical ectopic pregnancy, miscarriage in process, or cesarean scar pregnancies, all of which may carry significant morbidity and mortality 3.

A cervical ectopic pregnancy occurs when the blastocyst implants below the level of the internal cervical os resulting in a pregnancy in the endocervical canal 4. The uterus receives its blood supply from the uterine vessels which are under the broad ligament and connect with the uterus at the isthmus level, which is almost immediately next to the cervix 5. As the endocervical pregnancy grows, the cervical canal expands and the walls thin. If the pregnancy matures undiscovered, embryonic growth will lead to cervical canal rupture. When this happens, the risk of life-threatening bleeding is high due to the proximity of the uterine vessels. Gynecology should be consulted emergently when a cervical ectopic is identified as intervention can be both life and uterus saving.

Transvaginal ultrasound is the best way to look for cervical ectopic pregnancy if suspected. Characteristics seen on ultrasound include a barrel shaped uterus, a gestational sac located within the cervical canal (distal to the internal cervical os), and an absent sliding sign. The sliding sign is observed when the gestational sac inside the cervix fails to slide along the cervical canal wall when external pressure is applied. The presence of sliding is indicative of a miscarriage with fetal parts inside the cervix 6,7.

A cesarean scar ectopic is another concern when the pregnancy is identified in the lower uterine segment. This is identified on ultrasound when the uterus and cervical canal are empty and the gestational sac is implanted in the anterior portion of the lower uterine segment with no myometrium between the bladder and the gestational sac 8. If cesarean scar pregnancies are identified in the first trimester, there is decreased rate of hysterectomy9.

Case Resolution

The patient was ultimately seen by gynecology in the ED due to concern for a lower uterine segment pregnancy. They reviewed the imaging and diagnosed the patient with a missed abortion in the setting of a closed OS and a crown-rump length of 13mm with no cardiac activity. The patient decided to pursue expectant management instead of misoprostol or a dilation and curretage. She was discharged with plan for follow up in the gynecology clinic for re-evaluation.

Take Home Points

  • Both transabdominal and transvaginal ultrasound are used to evaluate the uterus and the adnexa to determine the location of an intrauterine pregnancy.
  • An IUP identified in the lower uterine segment of the uterus has important diagnostic and therapeutic implications given the potential for significant associated morbidity and mortality.
  • When a gestational sac or fetal parts are located within the cervical canal, the sliding sign can help differentiate cervical ectopic from a miscarriage.

Authored by Payton Leech, MD

Dr. Leech is a second year resident at the University of Cincinnati Emergency Medicine residency.

Faculty edits by Patrick Minges, MD

Dr. Minges is an an assistant professor of emergency medicine at the University of Cincinnati and is fellowship trained in Ultrasound.

References

  1. Abuhamad A. Ultrasound in Obstetrics and Gynecology: A Practical Approach. 2014. Chapter 4.

  2. “Lower Uterine Segment.” Concise Medical Dictionary, Oxford University Press, 2010.

  3. Sherer DM, Gorelick C, Dalloul M, Sokolovski M, Kheyman M, Kakamanu S, Abulafia O. Three dimensional sonographic findings of a cervical pregnancy. Journal of Ultrasound in Medicine. 2008; 27(1)

  4. Chukus A, Tirada N, Restrepo R, Reddy N. Uncommon implantation sites of ectopic pregnancy: thinking beyond the complex adnexal mass. Radiographical Society of North America. 2015; 35(3): 946-9: 946-959.

  5. Chaudhry R, Chaudhry K. Anatomy Abdomen and Pelvis, Uterine Arteries. StatPearls. 2019.

  6. Amato P. Diagnosis and Management of Cervical Ectopic Pregnancy. The foundation for excellence in Women’s Health. 2014. https://www.exxcellence.org/pearls-of-exxcellence/list-of-pearls/diagnosis-and-management-of-cervical-ectopic-pregnancy/

  7. Winder S, Reid S, Condous G. Ultrasound diagnosis of ectopic pregnancy. Australas Journal of Ultrasound Medicine. 2011; 14(2)29-33. 

  8. Weerakkody, Yuranga. “Cesarean Scar Ectopic Pregnancy: Radiology Reference Article.” Radiopaedia Blog RSS, radiopaedia.org/articles/caesarean-scar-ectopic-pregnancy?lang=us.

  9. Michaels A, et al. Outcome of cesarean scar pregnancies diagnosed sonographically in the first trimester. Journal of Ultrasound Medicine. 2015; 34(4).