Duration of labor. An analysis of influencing factors

9703 labors at Akershus Central Hospital during the period January 1979 to January 1984 were analysed to see which factors influenced the duration of labor. Median duration was 8.2 h for nulliparas, 5.3 h for multiparas. Parity greater than one had no influence on duration. Induced labors were 1.9 h (nulliparas) and 1.4 h (multiparas) shorter than those with spontaneous onset. Stepwise linear regression showed that duration of labor was positively correlated with the weight of the infant, duration of pregnancy, weight gain and prepregnant weight. It was negatively correlated with mother's height. Mother's age did not influence duration of labor. Occiput posterior presentation and extensions of the head prolonged labor in nulliparas, but not in multiparas, while breech presentation had no influence.