A gestational sac implanted in the niche of a previous caesarean scar, with thin myometrium between sac and bladder and peritrophoblastic flow — distinguish it from a low intrauterine sac or a miscarriage in progress, because it leads to accreta or rupture.
Orient first
- Incidence rises with caesarean rates; early diagnosis allows treatment before haemorrhage.
- The sac is in the anterior lower segment at the scar, with an empty cavity and cervical canal.
- Two types: on the scar (growing towards the cavity) and in the niche (growing towards the bladder) — the latter is more dangerous.
Acquire the study
- Transvaginal ultrasound with colour and spectral Doppler; sagittal plane through the scar; transabdominal with a full bladder for myometrial thickness.
The manoeuvre
- Sagittal plane: sac location relative to the scar and the internal os; empty uterine cavity and cervical canal.
- Myometrial thickness between sac and bladder in mm.
- Colour Doppler: high-velocity, low-resistance peritrophoblastic flow around the sac.
- Sliding sign: gentle probe pressure does not move the sac (unlike a miscarriage in progress).
- Embryo and cardiac activity; CRL in mm.
What confirms it
- A sac embedded in the scar niche with thin anterior myometrium and peritrophoblastic flow, with an empty cavity and cervix.
What licenses you to exclude it
- A sac that slides on probe pressure without flow is a miscarriage in progress, not a scar pregnancy.
The classic misread
- Calling a low intrauterine pregnancy normal without looking at the scar.