An ectopic pregnancy located in the interstitial segment of the fallopian tube. An especially dangerous form of tubal pregnancy, because the surrounding myometrium of the uterus supports the pregnancy's growth for weeks without causing symptoms; when rupture eventually occurs it can be catastrophic, because a main branch of the main artery to the uterus is torn, with sudden (sometimes fatal) bleeding into the peritoneal cavity.

A commonly used term for an interstitial pregnancy, but can also refer to an angular pregnancy, a pregnancy in a unicornuate uterus or a pregnancy in one side of a bicornuate uterus.