Placenta praevia is said to occur when a placenta is situated in the lower uterine segment. There are degrees of abnormality associated with increasing need for a caesarian section instead of SVD; these are:

• 1st - just reaching the lower segment (minor praevia)
• 2nd - reaches the internal cervical os
• 3rd - crosses the os (major praevia)
• 4th - completely crossing the os

The degree of placenta praevia is also dependent on the extent of cervical dilatation at the time of examination.

It occurs in about 0.5% of pregnancies, and it is associated with maternal antepartum haemorrhage.

Uterine contractions result in the shearing off of the placenta from the myometrium and the decidua. As a result, the closer to term, the larger the amount of bleeding.

NICE suggest that (1):

• because most low-lying placentas detected at the routine anomaly scan will resolve by the time the baby is born, only a woman whose placenta extends over the internal cervical os should be offered another transabdominal scan at 36 weeks. If the transabdominal scan is unclear, a transvaginal scan should be offered

• advice from the UK National Screening Committee on screening for placenta praevia...click here

Reference:

1. NICE (2008). Antenatal care