Clinical overview
Ectopic pregnancy is implantation of a fertilised ovum anywhere other than the uterine endometrial cavity. It complicates approximately 1.5–2% of all pregnancies, and remains a leading cause of first-trimester maternal mortality in South Africa and globally. Mortality is overwhelmingly from missed or delayed diagnosis resulting in rupture and haemorrhage. The clinical and management chapter is at Ectopic pregnancy management; here we cover the underlying biology — sites, aetiology, risk factors, and natural history. Understanding why ectopics happen and what governs their fate (rupture vs spontaneous resolution) underpins every management decision.
Core knowledge
Sites of implantation
Common ectopic implantation sites, including tubal, cervical, ovarian, and caesarean scar locations.
- Tubal (95–96%): the dominant site.
- Ampulla (70%) — widest part, often allows largest pre-rupture size.
- Isthmus (12%) — rupture earlier and more catastrophically because of narrow lumen and prominent muscle layer.
- Fimbria (11%) — sometimes presents as a tubal abortion (extrusion of pregnancy out the fimbrial end into the pouch of Douglas).
- Interstitial / cornual (2–3%) — within the intramural portion of the tube within the uterine wall; the most dangerous because rupture is later (8–12 weeks) and bleeding is catastrophic.
- Ovarian (~0.5%): rare; criteria (Spiegelberg) include the tube being intact and separate from the ovary on the affected side.
- Cervical (<0.5%): implantation in the cervical canal. Often diagnosed only at curettage with profuse bleeding. Risk factors: previous CS, prior cervical surgery, IVF.
- Scar (caesarean scar pregnancy, CSP) (~0.15%): rising incidence with rising CS rate. Implantation in the niche of a previous CS scar. Can be life-threatening from rupture or placenta accreta in continued pregnancy.
- Abdominal (~1% of ectopics): implantation on peritoneal surfaces (omentum, bowel, broad ligament). Often missed; can rarely progress to advanced gestation with anomalous fetus.
- Heterotopic: coexistence of intrauterine and ectopic pregnancy. ~1 in 30,000 spontaneous; ~1 in 100–200 in IVF — critical not to miss in ART patients.
Aetiology and risk factors
The unifying mechanism in tubal ectopics is impaired tubal transport of the fertilised ovum, allowing implantation in the tube rather than the uterine cavity.
Major risk factors:
- Prior pelvic inflammatory disease (PID) — particularly Chlamydia trachomatis. PID damages ciliated tubal epithelium, distorts lumen, creates adhesions. See Tubal infection pathology.
- Prior tubal surgery (sterilisation reversal, salpingostomy for prior ectopic).
- Previous ectopic pregnancy — 7–15% risk in next pregnancy.
- Tubal pathology on HSG (hydrosalpinx, distortion).
- In-utero DES exposure (historical).
- Assisted reproduction — particularly with tubal factor infertility.
- Smoking — alters tubal ciliary function dose-dependently.