Changes in plasma steroid hormones during treatment of preterm labour with ritodrine-HC1

Summary. Peripheral plasma concentrations of unconjugated oestradiol 17-β, progesterone and total oestriol were measured in 21 patients presenting with 'uncomplicated' preterm labour. Serial measurements were made during intravenous treatment with ritodrine over a period of 24 h. Mean levels of unconjugated oestradiol 17-β fell significantly during infusion with ritodrine. Changes in progesterone levels remained within the range of diurnal fluctuations normally found in uncomplicated late pregnancy. Mean total oestriol levels fell significantly, but the changes did not exceed normal variations. No correlation was found between the magnitude, the rate or the timing of changes in any of the hormones measured and the short or long term effects of ritodrine on uterine activity. Although alterations in the peripheral oestrogen concentrations may be a direct effect of β-mimetics, it is unlikely that this mechanism is important in the inhibition of uterine activity.