Localization of ectopic pregnancy
Ectopic pregnancy most frequently develops in the fallopian tubes, but other sites of gestational sac implantation are possible:
- Ampullary region of the fallopian tube (most common).
- Ovaries (about 5% of cases).
- Interstitial (cornual) segment of the fallopian tube.
- Cesarean scar (approximately 1%).
- Cervix and abdominal cavity (about 1.4%).
Bilateral localization is an extremely rare phenomenon (1:750–1,580). There is also heterotopic pregnancy (coexistence of intrauterine and ectopic pregnancies), which is observed in 1–3% of cases following assisted reproductive technologies (ART/IVF).
Ultrasound diagnostics
Ultrasound is the primary method for detecting ectopic pregnancy. The main diagnostic landmark is the absence of a gestational sac in the uterine cavity in the setting of a $\beta$-hCG level $\ge$ 2000 mIU/mL.
Key ultrasound features:
- Extrauterine mass: may contain a yolk sac or embryo. In 35% of cases, the mass is not visualized.
- Tubal ring / "ring of fire" sign ('tubal ring' / 'ring of fire'): a heterogeneous mass with a hyperechoic ring exhibiting increased vascularity. This sign is highly specific (99.5–100%), but sensitivity is only 20–65%.
- Adnexal mass: visualization of a structure located separately from the ovary (excluding simple cysts) is the most specific (98%) and sensitive (84%) criterion.
- Echogenic free fluid: its presence in the abdominal cavity with high probability (96% specificity) indicates the development of hemoperitoneum.
Features of specific forms
Certain types of ectopic pregnancy have a specific presentation and risks:
- Interstitial pregnancy: the gestational sac is located eccentrically within the endometrial cavity, and the thickness of the surrounding myometrium is $\le$ 5 mm. It manifests at a later gestation than standard tubal pregnancy. Rupture in this segment is accompanied by catastrophic hemorrhage.
- Cervical pregnancy (Cervical EP): the uterus acquires a "figure-of-eight" shape (figure of 8). In this case, differential diagnosis with an inevitable abortion—where products of conception simply prolapse through the cervical canal—is critically important.
MRI in diagnostics
Magnetic resonance imaging is rarely used and serves as a backup method. MRI is indicated in complex diagnostic situations, delayed presentation, or when hCG levels fail to rise.
On T2-weighted images, an ectopic pregnancy may appear as a heterogeneous adnexal mass. A characteristic feature is tortuous structures with low signal on T2—the "flow void" effect (flow voids) in large feeding vessels. This method helps distinguish a resolving ectopic pregnancy from other pelvic masses.
Briefly: answers to topical questions
Which adnexal conditions require differential diagnosis with ectopic pregnancy on ultrasound?
Differential diagnosis of ectopic pregnancy on ultrasound must be performed with corpus luteum cysts, hydrosalpinx, ovarian apoplexy, acute pelvic inflammatory disease, and torsion of an ovarian mass pedicle.
- Corpus luteum cyst (Corpus luteal cyst) — may demonstrate enhanced peripheral vascularity, mimicking an ectopic pregnancy.
- Hydrosalpinx (Hydrosalpinx) — requires differentiation from a progressive tubal pregnancy.
- Ovarian apoplexy, acute pelvic inflammatory disease, and torsion of an ovarian mass pedicle — differentiated from ruptured tubal pregnancy.
What direct and indirect ultrasound signs indicate a ruptured fallopian tube?
Signs of a ruptured tubal pregnancy on ultrasound may include:
- Free fluid in the abdominal cavity or pouch of Douglas corresponding to blood; echogenic free fluid indicates hemoperitoneum.
- Ill-defined heterogeneous mass in the adnexal region.
- Endometrial thickening, reflecting a decidual reaction.
- Absolute criterion for ectopic pregnancy — visualization of a gestational sac outside the uterine cavity.
Laparoscopy is the most accurate diagnostic method for a tubal abortion.
For cervical pregnancy, remember the figure-of-eight sign (figure of 8) — the uterus with a widened cervix visually resembles the number 8.
Class and exam questions
At what hCG level should the gestational sac be visible in the uterus?
At a $\beta$-hCG level $\ge$ 2000 mIU/mL, the absence of a gestational sac in the uterine cavity is suspicious for ectopic pregnancy.
What is the "ring of fire" sign?
This is an ultrasound sign (ring of fire) representing a heterogeneous mass with a hyperechoic ring and prominent blood flow around it. It is a highly specific sign of ectopic pregnancy.
What is the danger of interstitial pregnancy?
Due to its location within the myometrium (where the surrounding wall thickness is $\le$ 5 mm), it may develop longer than a tubal pregnancy, but its rupture leads to massive, life-threatening hemorrhage.
What else to study on this topic
- Placental morphological anomalies (bilobed, circumvallate, membranous)
- Placenta previa and low-lying placenta
- Placenta accreta spectrum (PAS) and diagnostic methods
- Umbilical cord insertion anomalies (marginal, velamentous)
- Vasa previa and associated risks
- Imaging findings of ovarian hyperstimulation syndrome
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