Objective: Well-trained women who continued training during pregnancy have shorter total delivery-time compared with a group who stopped training before the end of the first trimester (Clapp 1990). Less is known whether physical fitness in a general cohort of pregnant women is related to duration of delivery. Hence, the purpose was to measure maximal oxygen uptake (VO_{2 Max}) in late gestation and relate this to duration of delivery. Methods: About 4 weeks before delivery, 108 women (age 20– 40 yrs; 59 nulliparous and 49 multiparous), underwent a ramping cycle exercise test while connected to a Vmax 229 pulmonary function unit. Recordings were analysed following breath-to- breath measurements. Total duration of delivery was defined as the time between a 3 cm cervical dilation with regular uterine contractions and delivery. Results: We could not detect any significant differences between the nulli- and the multiparous groups for VO_{2 Max} or maximal power output. Total delivery-time was longer (P < 0.05) among the nulliparous compared with the multiparous women: 8.8 (4.7) and, 5.0 (3.6) hours, respectively. For nulliparous women who started the delivery spontaneously, the total delivery-time was inversely related to both VO_{2 Max} (P = 0.06; b?= ­4.56 hours²/Liter) and to maximal power output (P = 0.04; b?= ­0.06 hours/watt). These associations became even stronger when adjusted for birth weight and use of oxytocin during delivery. Conclusion: Pregnant multiparous women had significantly shorter total delivery-time compared with nulliparous, despite similar maximal oxygen uptakes and maximal power output at gestational week 36. Increased physical fitness was associated with decreased delivery time in nulliparous women. Reference: Clapp, JF. 1990. Am J Obstet Gynecol 163, 1799– 1805.