Prolonged labor at any stage is called dystocia. Dystocia is caused by a physical blockage or uterine inertia. The latter is a condition in which the uterus does not contract forcefully enough to expel the puppy. Dystocia usually affects the first puppy in the litter and is much more common in singleton litters.
Dystocia rarely occurs in healthy, well-conditioned dams. However, it does occur in fat, overweight brood bitches. This is one reason why it is so important to prevent excessive weight gain in pregnancy. Dystocia is common in breeds with large heads and broad shoulders, such as Bulldogs, Pugs, and Boston Terriers.