Normal pregnant women (N=300) were studied for SVC maneuver during all three trimesters. Respiratory frequency (Rf), tidal volume (TV), minute ventilation (VE), Inspiratory (Ti) and expiratory time (Te), duty cycle (Ti/Ttot) and inspiratory flow (TV/Ti) were studied longitudinally during pregnancy in different trimesters. Apparently healthy 100 nonpregnant women matched with age and socioeconomic status were studied as control. Ventilation increases initially and remains constant throughout the pregnancy. This increment in V_E during pregnancy is due to gradual and significant increase in TV (ANOVA <0.05), because Rf was unaltered throughout the pregnancy. Inspiratory time (Ti), flow (TV/Ti) and duty cycle found to be significantly higher in pregnant women when compare to nonpregnant control. Progesterone may effective centrally as respiratory drive maintained at high level throughout pregnancy. Progesterone is not effective locally at the level of respiratory apparatus because earlier study in same population suggest no changes in FVC manoeuvre.