Interstitial ectopic pregnancy

Last revised by Dr Francis Deng on 21 Nov 2021

Interstitial ectopic pregnancy, also known as intramural ectopic pregnancy, is an important type of ectopic pregnancy with higher risks of rupture and hemorrhage compared to usual tubal ectopic pregnancies

The term interstitial pregnancy is sometimes erroneously interchanged with cornual pregnancy, which specifically refers to the presence of an intrauterine gestational sac within a rudimentary uterine horn, a unicornuate uterus, cornua of a bicornuate uterus, or upper lateral portion of a septate uterus 1.

It accounts for 2-4% of all ectopic pregnancies and is the most common site for an atypical ectopic pregnancy. The incidence is thought to be rising 7.

It occurs within the interstitial portion of the fallopian tube and therefore has the potential to grow to larger sizes than standard tubal ectopic pregnancies by the time of presentation. Hence the risk and severity of hemorrhage are higher.

The main risk factor is a history of prior intrauterine instrumentation.

Pelvic sonography is the main method of diagnosis. 3D ultrasound may be helpful for delineating a gestational sac's location.

The main finding is an abnormally eccentric gestational sac and thin surrounding endo-myometrium or myometrium ("endomyometrial mantle"). The endomyometrial mantle measurement that indicates abnormal implantation varies in different studies but generally, <5 mm is highly suspicious for interstitial pregnancy while <8 mm is more sensitive but less specific 1.

An additional sonographic sign is the interstitial line sign 2, an echogenic line that runs from the endometrial echo complex to the interstitial mass/gestational sac. The premise is that the endometrium is separate from the interstitial pregnancy but is connected by the interstitial fallopian tube 6.

The gestational sac is located eccentric to the junctional zone.

The morbidity and mortality are higher (15x) due to a later presentation and associated complications. 

Management of an interstitial ectopic includes the use of methotrexate (either systemic or local), potassium chloride (KCl) injection, conservative laparoscopic surgery, uterine artery embolization (UAE), cornuectomy, or hysterectomy. The latter two are usually reserved for emergency situations or in case of failure of the other methods.

The main differential considerations are eccentric intrauterine pregnancy and cornual pregnancy. See the article on the differential diagnosis of an eccentric gestational sac.

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