Muscle respiratory strength studies during pregnancy are very scarce. The aim of this paper is to describe maximum inspiratory (PImax) and expiratory (PEmax) mean pressure values in women during their first pregnancy and to determine the relationship between the anthropometric, morphologic and physiologic variables of these pressures.One hundred and twenty women (120) primigravidas were studied from the 5th to 40th gestational week, ages ranging from 20 to 29 years old, euthrophic and with low risk pregnancies.PImax and PEmax mean values were 88.5 ± 16.52 cmH(2)O and 99.76 ± 18.19 cmH(2)O respectively. There was no association between gestational age and PImax (r = -0.06; p = 0.49) or PEmax (r = -0.11; p = 0.22). There was also no difference between PImax and PEmax during pregnancy trimesters and no correlation between pregnancy age and the pressures in each trimester. Height was the only anthropometric variable indicating a significant PImax (r = 0.20; p = 0.02) association. Fundal uterus height and inter-recti abdominis distance were not associated to respiratory pressure values. PEmax is not associated with the group of predictor variables (p = 0.127) and PImax demonstrated an independent association with height and dyspnea during physical exertion reflected by the following equation: PImax = 0.6 + 57.9 height - 1.68 dyspnea under effort. The present study suggests that inspiratory and expiratory maximum pressure values are not altered during different stages of pregnancy, however longitudinal studies are needed to assess changes over time.