Tubal ectopic pregnancy

US findings

US findings in ectopic pregnancy commonly include:

• The presence of an empty uterine cavity. Additionally, a pseudogestational sac or decidual cyst may be seen, representing a fluid collection centrally located within the endometrial canal, often mistaken for an early intrauterine pregnancy. Other uterine findings can include a decidual cast or a thick echogenic endometrium.
• A complex extra-adnexal mass or cystic structure surrounded by a thick echogenic ring, corresponding to the “tubal ring” sign. Doppler US may reveal increased vascularity around the ectopic mass- “ring of fire” sign. It's important to note that an absence of colour Doppler flow does not exclude an ectopic pregnancy. Hematosalpinx may be present. In rare cases, live extrauterine fetal cardiac activity may be detected.
• In the peritoneal cavity, free pelvic fluid or hemoperitoneum in the pouch of Douglas can also indicate an ectopic pregnancy.

Fig 1: Unruptured tubal ectopic pregnancy. Ultrasound image identifies ectopic gestational sac containing an embryo, in the right adnexal region, separate from the ovary. There is a thick hyperechogenic rim of tissue surrounding the gestational sac (A, arrow), also known as tubal ring sign. Both ovaries are normal, and the endometrial cavity is empty. There is small anechoic free fluid in pelvis (B, arrow). This case represents a classical unruptured tubal ectopic gestation with tubal ring sign, confirmed surgically.

Fig 2: Unruptured tubal ectopic pregnancy. The right adnexa reveal hyperechogenic rim of tissue surrounding the gestational sac, with peripheral vascularity, giving a "ring of fire" vascular pattern. Sac contains a living embryo.

CT findings

CT scans can detect more subtle findings. In the case of an unruptured ectopic pregnancy, CT reveals a prominently enhancing ring lesion in the adnexal region, corresponding to the "ring of fire" appearance seen on US. The mass may appear as a fluid-filled cyst, and no or minimal free fluid can be seen in the peritoneal cavity.

Uncommon ectopic pregnancies

Interstitial Pregnancy

This occurs when the pregnancy implants in the interstitial portion of the fallopian tube, which is the segment embedded within the uterine muscular wall. These pregnancies are rare, making up about 1-3% of all ectopic pregnancies. The risk factors are identical to other tubal pregnancies, except for ipsilateral salpingectomy, which specifically increases the risk of interstitial pregnancy. Interstitial pregnancies can be misdiagnosed as intrauterine due to partial implantation in the endometrium. Distinguishing an interstitial pregnancy from a normal intrauterine pregnancy with an eccentric position can be challenging on US. The diagnosis is suggested when the gestational sac is located high in the fundus, surrounded by less than 5 mm of myometrium. The "interstitial line sign," an echogenic line extending from the endometrium to the ectopic sac, is also helpful. On CT, a rounded, ring-enhancing mass may be seen adjacent to the uterine fundus in the cornual region, with partial myometrial coverage of the sac. Contrary to previous beliefs, interstitial pregnancy rupture can occur early in pregnancy.

Caesarean scar pregnancy

Caesarean scar pregnancy (CSP) occurs when a pregnancy implants within or on the scar of a previous cesarean delivery. Similar pregnancies can also occur in myomectomy scars, known as intramural pregnancies. In CSP, the gestational sac is located within the myometrium of the anterior lower uterine segment, at the site of the cesarean scar. A thin or absent layer of myometrium separating the gestational sac from the maternal bladder wall should be seen. Scar ectopic pregnancies are associated with a significantly higher risk of life-threatening hemorrhage and uterine wall rupture.

Cervical Pregnancy

A cervical pregnancy is a rare type of ectopic pregnancy where the trophoblast implants in the cervical tissue of the endocervical canal. A gestational sac is typically seen within the cervical stroma, often positioned eccentrically. This leads to cervical distention, causing an hourglass-shaped uterus.

Ovarian pregnancy

Ovarian pregnancy occurs up to 3 percent of ectopic pregnancies. US typically shows an adnexal mass or cyst with a hyperechoic outer ring, located within or on the ovary. Colour Doppler may reveal a hypervascular rim (ring of fire), although it can also appear in a corpus luteum cyst. Yolk sacs or embryos are rarely seen. When an adnexal mass is identified, an US examination can help differentiate its location as an intraovarian mass will move with the ovary under pressure. However, in a pregnant woman without visible intrauterine gestational sac, an anechoic or complex ovarian mass is more likely to be a corpus luteum cyst, as ovarian ectopic pregnancies are uncommon.

Abdominal pregnancy

An abdominal pregnancy is a rare type of ectopic pregnancy that can be primary, where the blastocyst implants directly on the peritoneal surface or abdominal organs, or secondary, resulting from the embryo's extrusion from the fallopian tube. It is most found in the pouch of Douglas (rectouterine space), but can occur anywhere within the peritoneal cavity, independent from the uterus, adnexa, and ovaries.

Heterotopic pregnancy

Heterotopic pregnancy occurs when there is an intrauterine pregnancy alongside an ectopic pregnancy. Although rare, the incidence of heterotopic pregnancies has increased with the emergence of assisted reproductive technologies.

Complications of ectopic pregnancy

Rupture

The most common complication of ectopic pregnancy is rupture. Rupture may lead to shock and if suspected, the patient requires immediate surgical intervention. In cases of ruptured ectopic pregnancy, the US may reveal a heterogeneous adnexal mass, often associated with echogenic free fluid, indicative of internal bleeding and can detect free peritoneal fluid and/or hemoperitoneum. However, this finding is not specific for ruptured ectopic pregnancies, as it can also be seen in intact tubal ectopic.

Fig 3: Ruptured ectopic pregnancy. Ultrasound image shows a large heterogenous collection in pelvis, corresponding to a haematoma. There is also present a mild quantity of free fluid (asterisk). This patient had high blood levels of β-HCG, and a ruptured tubal ectopic pregnancy was confirmed surgically.

Fig 4: Ruptured ectopic pregnancy. Ultrasound image shows thick solid material around the uterus and right adnexa, compliant with clotting haematoma (asterisk). Small quantity of free fluid is also present. Presence of intra-uterine device (arrow). This patient had high blood levels of β-HCG, and a ruptured tubal ectopic pregnancy was confirmed surgically.

CT typically shows a heterogeneous enhancing mass in the adnexal area, along with associated hemoperitoneum. Acute haemorrhage appears with an attenuation of 30–45 HU, while clotted blood is hyperdense (>60 HU). The “sentinel clot” sign indicates a hyperdense clot near the bleeding site, increasing confidence in identifying haemorrhage in the adnexa. Active haemorrhage can be seen as high-attenuation foci on contrast-enhanced CT, resembling adjacent vessels.

Fig 5: Ruptured tubal ectopic pregnancy. Non-contrast coronal CT image shows complex right adnexal mass, with central cystic area (arrow). Hyperdense fluid consistent with hemoperitoneum is observed extending along the paracolic gutters. Acute haemorrhage (orange asterisks) shows an attenuation of 30–45 HU, while clotted blood (red asterisks) is hyperdense, exceeding 60 HU. The sentinel clot sign indicates a hyperdense clot located near the bleeding site, which enhances the certainty of identifying haemorrhage in the adnexa. Ruptured tubal ectopic pregnancy was confirmed surgically.

Fig 6: Ruptured tubal ectopic pregnancy. Contrast-enhanced axial (A) and coronal (B) CT reveal a complex right adnexal mass with a small central cystic area, located around or within the right fallopian tube, showing peripheral enhancement (arrow). Adjacent hyperdense foci indicate active bleeding, likely from a rupture of the interstitial portion of the tube. Moderate amount of hyperdense free fluid (orange asterisk) extends into upper abdomen, representing hemoperitoneum. Clotted blood (red asterisk) surrounding the right adnexal mass, is consistent with the sentinel clot sign.

Fig 7: Ruptured tubal ectopic pregnancy. Contrast-enhanced axial (A) and coronal (B) CT images show contrast agent extravasation (orange arrow) surrounding or within right fallopian tube. There is a moderate amount of hyperdense free fluid (orange asterisk) indicative of acute haemorrhage, along with areas of higher attenuation (red asterisks), likely representing acute clotted haemorrhage.

Lithopedium

A lithopedium, or "stone baby", is a rare condition where a fetus dies during an ectopic pregnancy and calcifies within the abdominal cavity. Are often asymptomatic. On US, it appears as a calcified mass with fetal features. On CT, it shows a densely calcified, well-defined mass, often with surrounding inflammation.