In the management of a breech presentation, three factors must be considered in order to decide between vaginal delivery and cesarean section. These are (1) pelvic size, (2) pelvic shape, and (3) estimated fetal weight. Pelvic size can be accurately measured by x-ray pelvimetry. Unless the infant is premature, vaginal delivery is contraindicated in flat pelves with an obstetric conjugate of less than 10 cm. Fetal weight is best estimated by having four or more examiners give an estimate without the knowledge of others and averaging the estimates. In addition to the above three factors, the behavior of the sacrum and consideration of uterine inertia must also be taken into account.
Kaltreider DF. FETOPELVIC GRADING OF BREECH PRESENTATIONS. JAMA. 1957;165(2):132-135. doi:10.1001/jama.1957.02980200012004