### Description

The size of many placental lesions correlates with their clinical significance, with small lesions having little import while large lesions can have devastating effects on the developing fetus. A common way of expressing the size of a lesion is the percent of the total placental plate volume affected.

Most placentas are either circular or elliptical. Most lesions involving the plate are cylindrical or spherical.

area of an ellipse in square centimeters =

= π * (radius along long axis in cm) * (radius along short axis in cm) =

= π * ((diameter along long axis in cm) / 2) *  ((diameter along short axis in cm) / 2)

volume of the placental plate in cubic centimeters=

= (area of ellipse in square cm) * (thickness of plate in cm)

area of cylindrical lesion in square centimeters =

= π * ((radius of lesion in cm) ^2) =

= π * (((diameter of lesion in cm) /2 ) ^2)

volume of cylindrical lesion in cubic centimeters =

= π * ((radius of lesion in cm) ^2) * (thickness of lesion in cm) =

= π * (((diameter of lesion in cm) /2 ) ^2) * (thickness of lesion in cm)

area of  a circular lesion in square centimeters =

= π * ((radius of lesion in cm) ^2)

= π * (((diameter of lesion in cm) /2 ) ^2)

volume of spherical lesion in cubic centimeters =

= π * 4/3 * ((radius of lesion in cm) ^3)

= π * 4/3 * (((diameter of lesion in cm) /2 ) ^3)

percent involvement of plate cross-sectional area involved by the lesion =

= (area of lesion in square centimeters) / (area of plate in square centimeters) * 100

percent involvement of plate volume by the lesion =

= (volume of lesion in cubic centimeters) / (volume of plate in cubic centimeters) * 100

Interpretation:

• The significance of the degree of involvement varies with the lesion, but, in general, the larger the lesion the more likely that it can adversely affect the fetus.

Limitations:

• Placentas can occur in a wide range of shapes, for which the circle or ellipse may not be accurate representations

• The thickness of a plate may vary, especially around the periphery

• Lesions such as infarcts will shrink as they scar, reducing the original volume affected to a smaller size. This can be adjusted for by using the proportion of the thickness affected, or the proportion left unaffected.