Original classification based on placental palpation through the os:



Distance from the internal os to the placental edge. This group has reported that a placental edge located two cm or more from the internal os is not associated with intrapartum bleeding (using transvaginal sonography) (2).

We believe that all low lying placentas seen at routine 18-22 weeks scanning should be assessed by either transvaginal or translabial scanning to:

        Confirm the position of the placenta in relation to the internal os.

o       If the placenta is 2 cm or more from the os no follow up is necessary.

o       If the placenta is < 2 cm from the internal os follow up should be obtained at about 32 weeks.

o       If the placenta covers the internal os the distance that it projects beyond the os should be measured.

        Assess the attachment of the umbilical cord to the placenta (beware of velamentous insertion with vasa previa).

        To exclude placenta accreta / increta / percreta.




  1. Pauzner D, Barrett J, Farine D. Transvaginal scanning in the management of placenta previa. J SOGC 1995;17:231-235.
  2. Oppenheimer LW, Farine D, Ritchie JWK What is a low lying placenta? Am J Obstet Gynecol 1991;165(4):1036-1038.