- For pregnancies at more than 16 weeks of gestation the term low-lying placenta should be used when the placental edge is less than 20 mm from the internal os on transabdominal or transvaginal scanning (TVS).

If the distance between the internal os and the placental edge is 20 mm or more on TVS, the placental location should be recorded as normal and managed as per routine

• The term placenta praevia should be used when the placenta lies directly over the internal os.

If the placenta is thought to be low lying (less than 20 mm from the internal os) or praevia
(covering the os) at the routine fetal anomaly scan, a follow-up ultrasound examination
including a TVS is recommended at 32 weeks of gestation to diagnose persistent low-lying
placenta and/or placenta praevia.

Use of TVS- to accurately see the placental edge. Also, cervical length is a predictor of antepartum bleeding and emergency preterm caesarean section in placenta praevia. Women with a cervical length of less than 31 mm have a 16 times higher risk of emergency
caesarean section due to massive haemorrhage.