Pelvimetry

Background
Pelvimetry is the assessment of the female pelvis in relation to the birth of a baby. Traditional obstetrical services relied heavily on pelvimetry in the conduct of delivery in order to decide if natural or operative vaginal delivery was possible or if and when to use a cesarean section. With the increased safety of modern cesarean section and increased medico legal concerns about use of operative vaginal delivery, the threshold to perform a cesarean section has decreased and the need for pelvimetry diminished.

Use
Pelvimetry used to be performed routinely to discern if spontaneous labour was medically advisable. Women whose pelvises were deemed too small received caesarean sections instead of birthing naturally. Research indicates that pelvimetry is not a useful diagnostic tool for CPD (see below) and that in all cases spontaneous labour and birthing should be facilitated. See Blackadar & Viera, 2003, p505

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, head of bed 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. Since obstetricians continue to place women in this position for their requirement of 'access', not considering the birthing mother's needs to be in a better position to open her pelvis, it is more likely that women will be given the diagnosis that their pelvis is too small to birth their baby.

Cephalo-pelvic disproportion: CPD
Cephalo-pelvic disproportion 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 (see below) pelvic formation, or  a large fetus, and combinations of these. Certain medical conditions may distort pelvic bones, such as rickets or  a pelvic fracture, and lead to CPD.

Terminology
The terms used in pelvimetry are commonly used in obstetrics. Clinical pelvimetry attempts to assess the pelvis by clinical examination. Pelvimetry can also be done by radiography and MRI.

Pelvic planes

 * Pelvic inlet: The line between the narrowest bony points formed by the sacral promontory and the inner pubic arch is termed obstetrical conjugate: It should be 11.5 cm or more. This anteroposterior line at the inlet is 2 cm less than the diagonal conjugate (distance from undersurface of pubic arch to sacral promontory). The transverse diameter of the pelvic inlet measures 13.5 cm.
 * Midpelvis: The line between the narrowest bone points connects the ischial spines; it typically exceeds 12 cm.
 * Pelvic outlet: The distance between the ischial tuberosities (normally > 10 cm), and the angulation of the pubic arch.

Pelvic types
Traditional obstetrics characterizes four types of pelvises:
 * Gynecoid: Ideal shape, with round to slightly oval (obstetrical inlet slightly less transverse) inlet: best chances for normal vaginal delivery.
 * Android: triangular inlet, and prominent ischial spines, more angulated pubic arch.
 * Anthropoid: inlet transverse is greater than inlet obstetrical diameter.
 * Platypelloid: Flat inlet with shortened obstetrical diameter.

Fetal relationship

 * Engagement: The fetal is engaged if the widest leading part (typically the widest circumference of the head) is negotiating the inlet.
 * Station: Relationship of the leading bony part of the fetus to the maternal ischial spines. If at the level of the spines it is at “0(zero)” station, if it passed it by 2 cm it is at “+2” station.
 * Attitude: Relationship of fetal head to spine: flexed, neutral (“military”), or extended attitudes are possible.
 * Position: Relationship of presenting part to maternal pelvis, i.e. ROP=right occiput posterior, or LOA=left occiput anterior.
 * Presentation: Relationship between the leading fetal part and the pelvic inlet: cephalic, breech, or shoulder presentation.
 * Lie: Relationship between the longitudinal axis of fetus and mother: longitudinal, oblique, and transverse.
 * Caput or Caput succedaneum: edema typically formed by the tissue overlying the fetal skull during the vaginal birthing process.