In one line
A non-tubal ectopic implants where there is no muscle to contain it and no easy access to remove it, so it bleeds catastrophically and late — the consultant task is to locate it precisely before treating, match the treatment to the site, and never reach for the tubal reflex (systemic single-dose methotrexate or a simple salpingectomy) on a caesarean-scar, cervical, interstitial or abdominal pregnancy, where that reflex either fails or kills.
This chapter assumes the tubal groundwork — the discriminatory zone, the serial-hCG logic, single-dose methotrexate, salpingectomy versus salpingotomy. It covers the implantation sites that behave differently from the tube, where the literature is thin, where the guidelines diverge or fall silent, and where the wrong first move is irreversible.
Mechanism & pathophysiology
The danger of every non-tubal site follows from why the tube itself is dangerous. The tube has a thin muscular wall and no decidua, so trophoblast invades the wall directly, erodes vessels, and ruptures into the peritoneum once the conceptus outgrows the lumen. Every non-tubal site is a variation on that theme — trophoblast invading tissue that was never built to hold a pregnancy or to be operated on safely — and the danger of each site is predicted by two things: how vascular the bed is, and how little normal myometrium lies between the trophoblast and a free peritoneal or vaginal surface.
The caesarean-scar pregnancy is the one that has changed the specialty, because its incidence rises in lockstep with the caesarean rate. A caesarean leaves a hysterotomy scar that often heals with a wedge-shaped defect — the niche (isthmocele) — a pouch of deficient myometrium on the anterior lower segment. A blastocyst can implant into that niche or onto the scar, and because the niche has little or no muscle beneath it, invading trophoblast reaches the bladder and the uterine vessels early. The central concept follows: caesarean-scar pregnancy and placenta accreta spectrum are the same disease at different gestations. A scar pregnancy that is left to continue does not "move away" from the scar; it is an early accreta, and the deeper it sits in the niche the more certainly it becomes increta or percreta. The verified outcome data make this concrete — in the Kaelin Agten/Timor-Tritsch series, pregnancies implanted in the niche almost all came to cesarean-hysterectomy for placenta increta or percreta, whereas those on the scar (with residual myometrium beneath them) mostly delivered normally, and a first-trimester myometrial thickness under 2 mm predicted a placenta accreta spectrum at delivery.
The other sites each have a characteristic failure mode:
- Cervical pregnancy implants in the endocervical canal below the internal os. The cervix is fibrous with almost no contractile muscle, so it cannot clamp down on the placental bed — disturbing it (a curette, a "missed miscarriage" evacuation) opens sinusoids that will not constrict, producing torrential, sometimes unstoppable, haemorrhage.
- Interstitial pregnancy implants in the intramural segment of the tube as it traverses the uterine cornu. That segment is surrounded by myometrium, so it is distensible and ruptures late, commonly at 7 to 16 weeks and sometimes into the second trimester, by which time the conceptus is large and the cornual and ascending uterine vessels feed it, so rupture is a major obstetric haemorrhage, not a tubal trickle.
- Ovarian pregnancy implants in or on the ovary; the ovary is highly vascular and friable, and bleeding is usually early.
- Abdominal pregnancy implants on peritoneum, bowel, omentum or vessels; the placenta invades whatever it lands on, so the lethal moment is often at attempted removal of the placenta, not at diagnosis.
- Heterotopic pregnancy — a coexisting intrauterine and ectopic pregnancy — is the trap of the ART era: a positive scan showing an intrauterine sac falsely reassures, while the ectopic ruptures. Background incidence is roughly 1 in 30,000 spontaneously but of the order of 1 in 100–500 after IVF, so any assisted-conception pregnancy with pain, free fluid or an adnexal mass keeps heterotopic on the list even when the intrauterine pregnancy is seen.
One framework governs every management decision that follows: vascularity plus deficient myometrium equals danger, and the scar pregnancy is an early accreta.
Assessment
Getting the diagnosis: precise location, not "an ectopic"
The single most important investigation is a good transvaginal ultrasound that names the site, because "ectopic pregnancy" is not a diagnosis you can act on here — the treatment for a cervical pregnancy would kill in a scar pregnancy and vice versa. The mistakes are diagnostic before they are therapeutic.
The site-specific criteria a consultant must be able to state:
- Caesarean-scar pregnancy (SMFM criteria): an empty uterine cavity and empty endocervical canal; a gestational sac or placenta embedded in the hysterotomy scar; a triangular sac at ≤8 weeks (rounded/oval later) filling the niche; a thin (1–3 mm) or absent myometrial layer between sac and bladder; rich peritrophoblastic vascularity at the scar; and an embryo/yolk sac with or without cardiac activity. The discriminator from a cervical pregnancy and from a miscarriage in transit is that the sac sits anteriorly at the scar with the cavity and cervix empty.
- Cervical pregnancy: sac below a closed internal os, in the cervical canal, with cervical ballooning ("hourglass" uterus), trophoblastic flow on Doppler, and a negative "sliding sign" (the sac does not slide against the canal under probe pressure, unlike a miscarriage passing through).
- Interstitial pregnancy: an eccentric sac lying >1 cm lateral to the edge of the endometrial cavity, surrounded by a thin (<5 mm) myometrial mantle, with the "interstitial line sign" — an echogenic line from the cavity to the sac.
- Ovarian pregnancy: rarely diagnosed pre-operatively; classically a wide echogenic ring on the ovary. The historical Spiegelberg criteria define it pathologically — intact ipsilateral tube separate from the ovary, sac occupying the ovary, ovary connected to the uterus by the ovarian ligament, and ovarian tissue in the sac wall.
- Abdominal pregnancy: no myometrium surrounding the sac, sac separate from the uterus and tubes, with the placenta on a peritoneal surface; suspect it when the uterus is empty but a viable pregnancy is seen elsewhere.
MRI is the second-line problem-solver, not a screening tool. It earns its place when ultrasound cannot resolve a scar-versus-cervical question, when defining the depth of niche invasion and bladder involvement before surgery on a scar pregnancy, or when mapping placental and vascular relationships in an advanced abdominal pregnancy for theatre planning. It does not replace the transvaginal scan and should never delay treating an unstable woman.
Defining the site precisely — the classic exam distinctions
Three pairs of terms are routinely confused, and the confusion is dangerous:
- Interstitial versus cornual versus angular. Interstitial = implantation in the intramural (interstitial) segment of the tube — a true ectopic, outside the cavity, surrounded by myometrium. Cornual is properly reserved for a pregnancy in the rudimentary horn of a unicornuate (Müllerian-anomaly) uterus — a different entity with its own rupture risk, often needing horn excision. Angular is not an ectopic at all: it is an intrauterine pregnancy implanted in the lateral angle of the cavity, medial to the uterotubal junction and the round ligament — it can be viable and carried to term, though with a higher rate of malposition and abnormal placentation. Calling an angular pregnancy "interstitial" leads to destroying a viable intrauterine pregnancy; calling an interstitial pregnancy "angular" leads to a fatal cornual rupture. The discriminator is the position relative to the round ligament and the thickness and continuity of the surrounding myometrium.
- On the scar versus in the niche (caesarean-scar pregnancy): on the scar (endogenous/Type I) implants over the scar with residual myometrium beneath it and grows towards the cavity — better prognosis, occasionally compatible with a continuing pregnancy under intensive surveillance. In the niche (exogenous/Type II) implants deep in the dehiscent defect with little or no myometrium beneath, grows towards the bladder, and is the one that ruptures early and becomes percreta.