Measure the distance from the placental edge to the internal os on a transvaginal scan: covering the os is praevia, within 20 mm is low-lying; and look with colour Doppler for fetal vessels running over the os (vasa praevia).
Orient first
- Most low placentas at the 20-week scan move up; re-scan at 32 weeks (verify your local pathway).
- Praevia with a previous caesarean raises the risk of placenta accreta spectrum.
- Vasa praevia is associated with velamentous cord insertion, bilobed or succenturiate placentas and IVF.
Acquire the study
- Transvaginal ultrasound (safe in praevia) in the sagittal plane of the cervix; colour Doppler over the internal os at low scale.
The manoeuvre
- Sagittal transvaginal view of the internal os: distance from the leading placental edge in mm — or covering the os.
- Placental cord insertion: central, marginal or velamentous.
- Colour Doppler over the os: a vessel with a fetal-rate arterial spectral waveform crossing within 2 cm = vasa praevia.
- Accessory lobes and the vessels connecting them.
- With a prior caesarean: the scar and signs of accreta (loss of the clear zone, lacunae, bridging vessels).
What confirms it
- Placental edge covering the internal os on transvaginal scan; a fetal vessel over the os confirmed on spectral Doppler.
What licenses you to exclude it
- A placental edge more than 20 mm from the internal os on a transvaginal scan excludes a low-lying placenta at that gestation.
The classic misread
- Relying on a transabdominal scan (bladder filling distorts the os).
- Calling the marginal sinus or a funic presentation vasa praevia without spectral confirmation.