This is usually due to technical artifacts:

·         Overfull bladder- Squashes the lower uterine segment apparently elongating the cervical canal (normally 3.5-4 cm).

·         Focal myometrial contraction- Uterine wall >1.5 cm thick due Braxton Hicks contractions. Partially empty the bladder and rescan in 20—60 minutes, by which time the appearances should have changed.

TVS improves the accuracy of a placental location in relation to the internal os, particularly in the posterior placenta.