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 five centimetres (two 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 centimetres in labour, usually five centimetres each way, for the woman to give birth vaginally.
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.
Grade 3 or major
The placenta is implanted in the lower segment and part of the placenta covers some of the cervical opening.
Grade 4 also major
The placenta totally covers the woman's cervix.
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.
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