The definition of an ectopic pregnancy is a pregnancy outside the uterine cavity, not outside the uterus, as the interstitial pregnancy is still a uterine pregnancy. Diagnosis
Early diagnosis is important and today facilitated by the use of sonography and the quantitative human chorionic gonadotropin (hCG) assay. As in other cases of ectopic pregnancy, risk factors are:  previous tubal pregnancy, IVF therapy, tubal surgery, and a history of sexual infection. Typical symptoms of an interstitial pregnancy are the classic signs of ectopic pregnancy, namely abdominal pain and vaginal bleeding. Hemorrhagic shock is found in almost a quarter of patients. ; this explains the relatively high mortality rate. In pregnant patients, sonography is the primary method to make the diagnosis, even when patients have no symptoms. The paucity of myometrium around the gestational sac is diagnostic, while, in contrast, the angular pregnancy has at least 5 mm of myometrium on all of its sides. Ultrasonic criteria for the diagnosis include an empty uterine cavity, a gestational sac separate from the uterine cavity, and a myometrial thinning of less than 5 mm around the gestational sac; typically the interstitial line sign—an echogenic line from the endometrial cavity to the corner next to the gestational mass—is seen. MRI can be used particularly when it is important to distinguish between an interstitial and angular pregnancy. On average, the gestational age at presentation is about 7–8 weeks.
When a new pregnancy is diagnosed it is important to monitor the pregnancy by transvaginal sonography to assure that is it properly located, and that the surgically repaired area remains intact. Cesarean delivery is recommended to avoid uterine rupture during labor. Epidemiology
Interstitial pregnancies account for 2–4% of all tubal pregnancies, or for  1 in 2,500 to 5,000 live births. About one in fifty women with an interstitial pregnancy dies. Patients with an interstitial pregnancies have  a seven-times higher  mortality than those with ectopics in general. With the growing use of assisted reproductive technologies, the incidence of interstitial pregnancy is rising. References
External links

The Ectopic Pregnancy Trust - Information and support for those who have had the condition by a medically overseen and moderated UK based charity, recognised by the National Health Service (UK) Department of Health (UK) and Royal College of Obstetricians and Gynaecologists