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Background: Women are recommended to perform regular exercise during pregnancy but the impact of physical fitness on duration of gestation and miscarriage is inconsistent. In addition, a dose-response relation between the amount of weekly exercise and increased risk of miscarriage in early pregnancy has been observed. Previous studies have mostly used an epidemiologic method. Larger studies using careful measurement of physical fitness are needed. Besides physical fitness, maternal circulating concentrations of the hormone relaxin have been associated with decreased duration of gestation. Methods: A prospective cohort including 20 women with miscarriage and 460 women with spontaneous onset of labour, recruited from maternal health care centres in central Sweden, were examined in early pregnancy regarding estimated absolute peak oxygen uptake (V̇ O2 peak, est.) by cycle ergometer test, and maternal circulating serum relaxin concentrations. Results: Women with miscarriage displayed the highest level of absolute V̇ O2 peak, est. (2.61 l/min) and the lowest serum relaxin concentrations (640 ng/l). Among women with spontaneous onset of labour, the mean absolute V̇ O2 peak, est. increased successively from the lowest estimated oxygen uptake of 2.31 l/min among those with preterm birth (n = 28), to an oxygen uptake of 2.49 l/min among women with postterm birth (n = 31). An opposite trend was shown regarding serum relaxin concentrations from women with miscarriage to those with postterm birth. Serum relaxin concentrations, but not absolute V̇ O2 peak, est. was significantly and independently associated with duration of gestation in women with miscarriages, and absolute V̇ O2 peak, est., age and multiple pregnancy were independently associated with duration of gestation in women with spontaneous onset of labour. Conclusions: Physical fitness appears to be a protective factor of established pregnancies and not significantly involved in the risk of early miscarriage. Additional studies are needed to more clearly define the role of relaxin in miscarriage. [ABSTRACT FROM AUTHOR]