Interstitial ectopic pregnancy occurs when the fertilized ovum implants in the interstitial portion of the fallopian tube, where it transverses the wall of the uterus from the cornua to the corner of the uterine cavity. Interstitial ectopic pregnancy is an infrequent type of ectopic pregnancy, occurring in 2% to 4% of all tubal pregnancies. It is a life-threatening condition because myometrium rupture tends to occur in the second trimester of pregnancy resulting in profuse bleeding. This paper concerns the ultrasonographic diagnosis of a 33-year-old, gravida 4, para 3 woman with unruptured interstitial pregnancy, diagnosed in the 7th week of pregnancy. Awareness and suspicion of such pregnancy are required for its early detection. A combination of the sensitive beta-hCG assays, a single serum progesterone measurement and transvaginal ultrasonography are the principle tools used to make the diagnosis. The diagnostic difficulties are also discussed.