Radiological Estimation of Pelvic Expansion

Abstract

It has long been recognized that the usual obstetric patient will deliver her second and third babies with greater ease and with shorter labor than her first. This is true even though customarily the birth weight of each succeeding child shows a slight increase over that of its predecessor. The decrease in dystocia in each succeeding delivery has in the past been attributed to the dilatation and lacerations of the soft parts of the birth canal, the lower uterine segment, cervix, vagina, and perineum. On this basis one would expect all multiparous deliveries, without positional dystocia, to be easier, provided the contractions are efficient and the increase in the size of the baby not excessive. However, an appreciable percentage of primiparous and multiparous second stage labors resemble each other closely. It is, therefore, necessary to consider that other factors may be responsible for inconsistent prognostications of multiparous deliveries.Heretofore the incidence, degree, and clinical significance of pelvic expansion during pregnancy and labor have never been accurately estimated. Allen 1 has noted that the usefulness of pelvimetry at the midplane level is limited, owing to the physiological increases in the diameters possible during labor. Moir and others 2 have stated that, if pelvic measurements are inadequate in the cavity or outlet only, some 25% of easy deliveries may occur, especially in multíparas, because of pelvic expansion. Allen has further remarked that at the midplane the difficulties of deciding whether vaginal delivery is possible are even greater than at the inlet. "This is partly because of greater moulding of the skull and partly because the diameters at these lower levels are capable of physiological increase during labor." Stander (1945), for example, considers that an exaggerated Sims posi¬

Bony birth‐canal dimensions and obstetric constraints in hominoids

“…In modern humans, fetal‐maternal craniopelvic disproportion is addressed differently: partly through a much more limited relaxation of the mother's sacroiliac and interpubic ligaments (Marnach et al, 2003; Weinberg, 1954), and partly by distortion of the fetal skull (Becker et al, 2010; Falk et al, 2012; Pires et al, 2016; Tague, 2012; Vleeming et al, 2008). Human neonates have unfused cranial bones that can be displaced by birth‐canal pressures to produce a reduced fetal skull diameter.…”