Placenta previa is medically categorised into 4 grades of severity. These are:
Grade 1 or minor.
Where part of the placenta is implanted in the lower segment of the uterus, but the edge is more than 5cms (2 inches) away from the woman's cervix. Most of the placenta is situated normally, in the upper segment of the uterus. The cervix needs to dilate 10 cms in labour, usually 5cms each way, for the woman to give birth vaginally.
Image 4-42 shows a Grade 1 Placenta Previa.
Grade 2 also minor.
Most of the placenta is implanted in the lower segment, and the edge of the placenta touches the edge of the cervical opening, but does not cover it.
Image 4-43 shows a Grade 2 Placenta Previa.
Grade 3 or major.
The placenta is implanted in the lower segment and part of the placenta covers some of the cervical opening.
Image 4-44 shows a Grade 3 Placenta Previa.
Grade 4 also major.
The placenta totally covers the woman's cervix.
Image 4-45 shows a Grade 4 Placenta Previa.
There is another type of placenta previa, where most of the placental tissue is situated in the upper segment, but there is a small piece of placenta that implants away from the main body of the placenta, over or near the cervix. This is a satellite lobe or succenturiate lobe and is connected to the main placenta by a large blood vessel running through the sac or membrane . This is rare and may be difficult to detect on ultrasound. You can read more here.
Image 4-46 shows a placenta with a Succenturiate lobe.
Updated August 2006
Gabbe SG, Niebyl, JR, Simpson JL. Obstetrics, Normal and problem pregnancies, 4th Edition, 2001, Churchill Livingstone, New York.
Stables D. and Rankin J. Physiology in Childbearing with Anatomy and Related Biosciences. 2004, Bailliere Tindall, Edinburgh.