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.
Weinberg A. RADIOLOGICAL ESTIMATION OF PELVIC EXPANSION. JAMA. 1954;154(10):822-823. doi:10.1001/jama.1954.02940440020005