Plasma oxytocin and oxytocinase levels in third trimester of pregnancy and at labour.

In women during the third trimester of normal pregnancy and at labour the level of oxytocin and progesterone in plasma were determined by means of radioimmunoassay and the activity of oxytocinase was measured spectrophotometrically. The plasma oxytocin levels measured from the 25th week of pregnancy showed a gradual increase with the peak values at the 39th and 40th week. At labour, the oxytocin values remain at the same level as these found at the end of pregnancy. The increase in oxytocin levels at contractions compared to its values found between contractions was not statistically significant. Oxytocinase levels showed also an ascending character until the 38th week of pregnancy. Later, a decrease was found which continued until the termination of pregnancy. At labour, there was a decrease in progesterone level in plasma which was about at the limit of significance.