CPD is best assessed by
Methods of assesment of Pelvis 

Clinically
Imaging studies

Clinical Pelvimetry :

Done manually
Can be done beyond 37th week, better done after onset of labour
Following are to be observed 


Sacrum 
Sacrosciatic notch
Iscial spines 
Side walls
Pubic arch
Pubic angle
Transverse diameter of outlet

Imaging :

X-ray pelvimetry 
USG
CT 
MRI

X-ray pelvimetry : 

Limited value in diagnosis
Can only assess size and shape of pelvis 
Useful in cases with pelvic fractures 
Disadvantage : Radiation exposure to both mother and foetus (885 milirad)
Erect lateral view 

CT 

Easier to perform
Radiation exposure(45-425milirad)
More accurate

MRI

More accurate to assess bony pelvis
Assess pelvic capacity in different planes
Assess foetal size and head volume, pelvic soft tissues
No radiation risk
Expensive,limited availability, more time taking

USG : 

Accurately measures the biparietal diameter
CT scan
Ultrasound
Radio pelvimetry
Pelvic assessment