Table 2 The fetal-pelvic index [32] and cephalopelvic disproportion index [33]

Fetal-Pelvic Index

Combines 4 measurements of the fetal head circumference and abdominal circumference with the maternal pelvic inlet and outlet measurements.

The sum of the two most positive fetal-pelvic circumference differences

A positive FPI should identify fetuses larger than the maternal pelvis

A negative FPI should identify fetuses smaller than the maternal pelvis

Calculation: HC-IC, HC-MC, AC-IC, AC-MC

Cephalopelvic Disproportion Index

The smallest pelvic diameter (either the anteroposterior of the inlet or the bispinal of the midpelvis) compared to the biparietal diameter of the fetal head

The difference between the two indicates how much wider the smallest diameter of the bony pelvis is than the fetal skull

A positive index is present if the pelvic diameter is less than 9 mm wider than the biparietal diameter.