Placenta Previa on USG

My Registrar informed me on ward round that one particular patient at term presented with antepartum hemorrhage. Her ultrasonography revealed the placental edge to be 2.45 cm from the internal os. "So is that placenta previa or is it not?" I asked. "It is not" she said after deep thinking for six seconds. "Find out the correct distance from the internal os, beyond which it is not placenta previa at term" I said. "In the meantime treat as type I placenta previa." That gave me an idea. I decided to conduct a survey, using one question. I called my four Assistant Professors (formerly called lecturers) and asked them the same question independently. The answers were 2.5 cm, 3 cm, 5 cm and 2.5 cm. Then I talked to all the Assistant Professors in the department and asked the same question. The answers ranged from 2.5 cm to 6.75 cm. I was amazed that on such a basic issue there was no consensus. I do not perform ultrasonography myself. But I know that the lower segment lies below the line at which the peritoneum on the anterior surface of the uterus ceases to be firmly adherent to the uterus and gets reflected onto the urinary bladder. Thus when the urinary bladder is full, if the placenta is found to extend below the upper limit of the urinary bladder anteriorly, it is previa. The distance between the internal os and the upper limit of the urinary bladder is the length of the lower segment in that woman at that gestational age. It can then be applied laterally and posteriorly to diagnose lateral or posterior type I placenta previas. I think the term 'low lying placenta - not previa' is superfluous, because if the placenta is not previa, it does not matter whether it is lower or higher.