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Background: Women are recommended to perform regular exercise during pregnancy but the impact of physical fitness on duration of gestation including miscarriage is inconsistent. In addition, an increased risk of miscarriage in early pregnancy among women with higher levels of physical activities has been noted. Previous studies have mostly used an epidemiologic method. Larger studies using careful measurement of physical fitness are needed. Besides physical fitness, elevated maternal circulating levels 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 start of labour, recruited from maternal health care centres in central Sweden, were examined regarding estimated absolute peak oxygen uptake (V̇O2 peak, est..) by cycle ergometer test, and maternal circulating relaxin levels in early pregnancy. Results: Women with miscarriage displayed the highest level of absolute V̇O2 peak, est. (2.61 l/min) and the lowest serum relaxin levels (640 ng/l). Among women with spontaneous start of labour, the mean absolute V̇O2 peak, est. increased successively from the lowest level (2.31 l/min) among those with preterm birth (n=28), to 2.49 l/min among women with post term birth (n=31). An opposite trend was shown regarding serum relaxin levels from women with miscarriage to those with post term birth. Serum relaxin concentrations, but not absolute V̇O2 peak, est. was significantly and independently associated with duration of gestation in women with miscarriages, and absolute V̇O2 peak, est., age and multiple pregnancy were independently associated with duration of gestation in women with spontaneous start 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.