The relation between plasma unconjugated oestriol and cystine aminopeptidase concentrations and uterine contractions.

The relation between maternal plasma unconjugated oestriol (E3) concentrations and uterine activity was investigated by very frequent, carefully timed determinations of E3 during synchronously recorded uterine contractions in six normal subjects. There was an initial rise (10 to 50 per cent above the mean) in E3 levels reaching a peak coincident with the contraction peak (range 20 seconds before to 15 seconds after) followed by a fall of similar magnitude below the mean reaching a nadir about 80 seconds (range 60 to 105 seconds) after the contraction peak. Although the pattern of E3 fluctuation was most marked in labour the overall degree of variation was no greater than at other times. Cystine aminopeptidase (CAP) concentrations in the same serum samples showed no significant correlation with uterine contractions. The reasons for, and practical applications of, the E3 results are discussed.