Objective To record any physiological changes in lung function during healthy pregnancies, and evaluate the influence of parity, pregestational overweight, and excessive weight gain.Design Longitudinal cohort study.Setting Antenatal clinic at Oslo University Hospital.Population One hundred healthy white women with singleton pregnancies.Methods The women were studied with repeated measures of lung function using spirometry at a gestational age of 14-16, 22-24, 30-32, and 36 weeks, and at 6 months postpartum.Main outcome measures Forced vital capacity (FVC), forced expiratory volume in 1 second (FEV1), and peak expiratory flow (PEF), also expressed as a percentage of predicted values according to age and height: i.e. FVC%, FEV1%, and PEF%.Results Both FVC and FVC% increased significantly after 14-16 weeks of gestation (P = 0.001), as was the case for both PEF and PEF% (P < 0.001). FVC, FVC%, PEF, and PEF% in early and mid-pregnancy were significantly lower compared with the postpartum value (all P < 0.05). Nulliparous women had an overall 4.4% lower value of FVC% than parous women (P = 0.039). There were no differences in FVC, FEV1, or PEF dependent upon pregestational overweight or excessive weight gain.Conclusions Forced vital capacity (FVC) increases significantly after 14-16 weeks of gestation. The FVC% is significantly higher in parous compared with primigravida women, suggesting that the changes in FVC occurring during pregnancy persist postpartum. PEF increases significantly during healthy pregnancies, and should be interpreted cautiously in pregnant women with impaired lung function.