In the present study the partial pressure of oxygen in arterial blood (PaO2) was estimated in 32 women in Dhaka city. For this purpose a total 32 women subjects with age ranged from 25 years to 35 years without any recent history of respiratory diseases were selected and divided into 8 healthy non-pregnant women as control and 24 normal pregnant women as experimental group. Experimental group was further divided into 8 first trimester, 8 second trimester & 8 third trimester of pregnant women. The PaO2 was estimated by using "Easy Blood Gas Auto Analyzer". The PaO2 was estimated during the 1st, 2nd, 3rd trimesters of pregnant women and non-pregnant women. The mean of measured values of PaO2 were analyzed statistically in relation to 1st, 2nd, 3rd trimesters of pregnant women & non-pregnant women. The PaO2 was significantly higher in 1st trimester, 2nd trimester & 3rd trimester of pregnant women than that of non-pregnant women. There were no statistically significant difference of PaO2 between the 1st trimester and 2nd trimester; between the 1st trimester and 3rd trimester; and between the 2nd trimester and 3rd trimester of pregnant women. It may be concluded that the causes of progressively increased PaO2 throughout the pregnancy were most likely to be due to effect of progesterone causing hyperventilation.