This work studies the changes in the acid base balance from the beginning to the end of expulsive stage of 73 labours (52 primiparas and 21 multiparas). These are typed according to the CTG patterns proposed by Melchior. All the pregnancies were normal and at or near term. The variation in pH measured is not the same in the primiparas as in the multiparas. For the multiparas the short stage of expulsion results in little change in the acid base balance, and with little correlation to the CTG type. Conversely for the primiparas the lowering of the pH measured corresponds well to the CTG type. For the type 0 it is 0.04 units of pH for the entire expulsive stage. This corresponds to a fall of -0.004 units of pH for each contraction. For the type 2 the fall is -0.12 units of pH or -0.02 per contraction. For the type 1 (the most frequent pattern seen, 71%) the fall is 0.1 units of pH, or -0.01 per contraction. For this last type of CTG pattern characterized by decelerations synchronous with the contractions, and considered for a long time as innocuous, these figures should alert the obstetrician to not permit the expulsive stage of labour to become unduly long. Finally the use of epidural anesthesia did not modify the fall of pH in this last group.