It is established that the level of physical activity in the population generally is too low, and data indicate that pregnant women are no exception. Studies of the effects of aerobic fitness on delivery outcomes are limited. In this observational study, we investigated the effect of aerobic fitness on duration of labor in nulliparous women who started labor spontaneously. Maximal oxygen uptake was measured in 40 nulliparous women at 35-37 weeks of gestation. Duration of labor was defined as the time between 3 cm cervical dilation with regular uterine contractions and delivery. The mean (SD) maximal oxygen uptake was 2.1 (0.3) L/min and duration of labor 583 (317) minutes. Duration of labor was inversely associated with maximal oxygen uptake after adjusting for birthweight (p = 0.034). We conclude that measurement of maximal oxygen uptake is safe in pregnancy at 35-37 weeks and that increased aerobic fitness was associated with shorter labor in nulliparous women who started labor spontaneously.