The implantation of the conceptus in the proximal portion of the Fallopian tube, which is within the muscular wall of the uterus, is called an interstitial pregnancy. The incidence of interstitial ectopic is 1 in 2500-5000 live births and it accounts for 2-6% of all ectopic pregnancies . Risk factors predisposing to an interstitial pregnancy are the same as those for tubal ectopics and include previous ectopic pregnancy (40.6%), assisted reproduction treatment (37.5%) and sexually transmitted infections (25%) . A unique predisposing factor to interstitial pregnancy is previous ipsilateral salpingectomy.
The maternal morbidity associated with interstitial pregnancy is still high, and the maternal mortality rate of this form of ectopic pregnancy is about 2-2.5% .
Interstitial pregnancy remains the most difficult type of ectopic pregnancies to diagnose pre-operatively. This is partly due to lack of any symptoms prior to sudden rupture. In modern clinical practice the diagnosis of interstitial pregnancy should be made non-invasively using transvaginal ultrasound. The diagnosis is based on the visualization of the interstitial tube adjoining the lateral aspect of the uterine cavity and the gestational sac, and the presence of a continuous myometrial layer surrounding the chorionic sac  (Figs 14.6 and 14.7).
Ruptured interstitial pregnancy usually presents dramatically with severe intra-abdominal bleeding, which requires urgent surgery. Haemostasis can usually be achieved by removing the pregnancy tissue and suturing the rupture site. However, in cases of extreme bleeding a cornual resection or in rare cases a hysterectomy may be necessary to arrest the bleeding.
Gestational sac Myometrium
Gestational sac Myometrium
Fig. 14.7 An oblique section through the uterus showing an empty uterine cavity and the interstitial portion of the tube adjoining the cavity and an ectopic sac. The sac is completely surrounded by a myometrial mantle, which is typical of interstitial pregnancy.
Unruptured interstitial pregnancy <12 weeks in size can be managed conservatively. Medical treatment with methotrexate should be given to all women with rising serum hCG on follow-up visit. Good results have been reported with both systemic and local methotrexate [46,47]. However, in viable interstitial pregnancies local injection under ultrasound guidance is preferable as it enables fetocide to be carried out at the same time, which increases the success rate of medical treatment. Small interstitial pregnancies with declining serum hCG levels can be managed expectantly without any intervention.
Apart from the side effects of methotrexate, the main disadvantage of conservative treatment is the time taken for the pregnancy to be fully absorbed and in larger pregnancies this may take up to a year.
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