Click on the links below.
The following may cause difficulties with accurate diagnosis of placenta praevia:
Posterior placenta
The baby may obscure the lower limit of the placenta.
Overfilled bladder
If the bladder is too full, it may compress the lower part of the uterus and make the cervix appear longer and therefore closer to the placenta.
Presenting fetal part (head) obscuring internal os
The fetal head in particular can compress the placenta and may make it difficult to see the os. You may need to try to move the baby, for example by raising the foot of the couch.
Underfilled urinary bladder
If the maternal bladder is underfilled, you may not see the internal os.
Lateral placenta
It may be easy to miss a lateral placenta extending close to the os if you only scan in the midline.