Pelvimetry is the assessment of metrics of the female pelvis as it pertains to the delivery of a baby. Traditional obstetrical services relied heavily on pelvimetry to decide if natural or operative vaginal delivery was possible or if and when to use a cesarean section. Pelvimetry used to be performed routinely to discern if spontaneous labor was medically advisable. Women whose pelvises were deemed too small received caesarean sections instead of birthing naturally. Clinical pelvimetry attempts to assess the pelvis by clinical examination. Pelvimetry can also be done by radiography and MRI.
Cephalo-pelvic disproportion (CPD) exists when the capacity of the pelvis is inadequate to allow the fetus to negotiate the birth canal. This may be due to a small pelvis, a nongynecoid pelvic formation, a large fetus, or a combination of these factors. Certain medical conditions may distort pelvic bones (such as rickets or a pelvic fracture) and lead to CPD. Transverse diagonal measurement has been proposed as a predictive method to determine pelvic capacity.
Pelvimetry used to be performed routinely to discern if spontaneous labor was medically advisable. Research indicates that pelvimetry is not a useful diagnostic tool for CPD and that in all cases spontaneous labor and birthing should be facilitated.
A woman's pelvis loosens up before birth (with the help of hormones), and an upright and/or squatting woman can birth a considerably larger baby. A woman in the lithotomy (lying on her back, legs elevated) is more than likely not going to push a larger than average baby out, due to the size of outlet that this position creates.