The current system of describing fetal position such as LOA, ROP, SA, etc. does not always give us all the information needed to decide if a baby is in the best position. Particularly with cephalic presentations, the distance that the denominator, such as the occiput, is from the center of the pelvis, or how close it is to the pelvic wall can mean the difference between a good position and a malpresentation. Another system, taken from mathematics may well serve us better, and if doctors will remember back to the higher math classes that they took once upon a time, we will also be able to communicate with them using this system.
The polar coordinate system uses two coordinates. One is the radius, or distance from the origin or center. The other is the angle of rotation from 0o. (Zero degrees corresponds to the Left Transverse Position.) These coordinates are written as (r, theta) where r is the radius and theta, is the angle measurement from 0o. A combination can also be used where one mentions the radius along with the established system of LOA, ROP, etc.
The reason this system is needed is that the distance from the center of the pelvis to the posterior fontanelle corresponds to the amount of flexion that has taken place. If we hear that a baby is in the OA position, this sounds like a good thing. It is only good if the radius is near 3cm. If the radius is at 5cm - right under the pubic bone - the baby’s presentation is a military attitude. Unless flexion or rotation takes place the baby will not be born vaginally. It also gives us language to describe what needs to change before vaginal delivery can be accomplished. So if you call a senior midwife in the middle of the night to describe precisely the position, she can tell you that you need to get the baby into such and such a position by doing thus and such.
If you have a copy of Human Labor & Birth, I can give you some examples of how this mapping system works. Look at figure 8 in chapter 10 of Human Labor & Birth. In the 5th edition this is the "summary of mechanism of labor: LOA." The following chart names some of the positions using both systems.
Human Labor and Birth
Using Polar Coordinates
Onset of Labor
r=5, theta=50 degrees
Descent and Flexion
r=3, theta=50 degrees
Internal Rotation: LOA to OA
r=2.5, theta=90 degrees
If you would like to print off a FREE form to document fetal presentation during labor,