Method for monitoring plasma progesterone concentrations in pregnancy

We present a time-series model for monitoring concentrations in plasma of hormones produced in the placenta, progesterone being chosen as an example. The model, which is based on the assumption that variations in plasma progesterone concentration in pregnant subjects mainly reflect variations in the growth rate of the placenta, was applied to eight series of progesterone values measured during pregnancy in eight subjects. In the model, which was found to fit the data, it is assumed that progesterone concentration is proportional to the size of the placenta and that the growth rate of the placenta varies at random, with a mean value alpha. The variation of alpha was of the same magnitude among and within the subjects. If the average of many subjects alpha values is used, a single subject may be used as her own reference, based on only one previous observation. When two observations are available, an individual's own alpha value may be estimated and used for the prediction. The predictive power of the new method was found to be far superior to the conventional method in which a single sample reference material is used. Furthermore, one need not know the gestational age in order to use the method.