Objectives To assess physical fitness in pregnancy and to evaluate its effect on perceived health, back pain, blood pressure and duration of gestation. Also, to evaluate the effect of serum relaxin levels on blood pressure and duration of gestation. Material and methods A prospective cohort of 520 pregnant women were examined in early pregnancy and five months postpartum with regard to socio-demographic characteristics and estimated peak oxygen uptake (V̇O2 peak, est.). Serum concentrations of relaxin were carried out in early pregnancy. Physical exercise, possible back pain and blood pressure were measured repeatedly throughout pregnancy. Results Absolute V̇O2 peak, est. in early pregnancy was positively correlated to perceived health, which was lower during than after pregnancy. The average absolute V̇O2 peak, est. in early pregnancy of 2.4 l/minute was 0.02 l/minute less than the V̇O2 peak, est. postpartum, while regular physical exercise decreased throughout pregnancy. Absolute V̇O2 peak, est. in early pregnancy was not associated to the incidence of any low back pain location in pregnancy or postpartum, but inversely to intensity of back pain and diastolic blood pressure and positively with duration of gestation. Elevated serum relaxin levels were associated with decreased diastolic blood pressure and higher duration of gestation among women with miscarriage. Conclusions Perceived health, diastolic blood pressure and duration of gestation were positively affected by physical fitness while no effect was shown on the incidence of back pain. The effect of physical fitness on duration of gestation and diastolic blood pressure might have clinical implications as well as the increased serum relaxin levels on miscarriages.