Another reason why multiparous women with prolonged or obstructed labours are more at risk of uterine rupture relates to the fact that they continue experiencing powerful labour contractions for much longer than first-time mothers.
In primiparous women, the uterine contractions remain relatively strong only for about the first 24 hours of labour, after which the contractions become weaker in intensity and shorter in duration. After about 36 hours, in the majority of primiparous women, the uterus is exhausted and they develop uterine inertia, which is when the contractions become very weak in intensity, with a short duration and long intervals between them. For such first-time mothers, because uterine contractions have almost ceased, uterine rupture is a rare phenomenon. By contrast, the risk to multiparous women whose labour is obstructed is that the uterine contractions remain forceful and frequent for very much longer, and as a result the uterus is more likely to rupture.
Primiparous women do face other serious problems, however, because uterine inertia means that the fetal head will stay in the maternal pelvis for a long time. This increases the risk of fetal hypoxia (oxygen shortage), and fistula formation, retention of urine and infection in the obstructed bladder of the mother.