Changes in rates of salivary estriol increases before parturition at term

The aim of this study was to characterize the increases of salivary estriol concentrations before the onset of labor at term.

Salivary estriol concentrations were measured in weekly patient-collected samples by means of a sensitive (mean +/- SD threshold, 0.025 +/- 0.001 ng/mL; coefficient of variation, 3.8%) direct enzyme immunoassay in a microtiter plate format. The salivary estriol concentrations in 16 healthy pregnant women were characterized from 30 weeks' gestation until the time of parturition and delivery. Samples were stored frozen at collection and analyzed in batches after delivery.

The median salivary estriol concentration profile revealed a nonlinear rise beginning from 30 weeks' gestation (0.89 ng/mL) until term (2.70 ng/mL, an increase of 201%). At 35 weeks' gestation the salivary estriol concentration median value increased sharply (positive inflection point, 50%-93% increase) at a demarcation between a slower increase during early pregnancy and a more rapid increase during late pregnancy. This positive inflection point associated with a late pregnancy increase characterized subgroups of pregnancies according to the lengths of gestation as follows: early term (delivered at <38 weeks 1 day's gestation), middle term (delivered at 38 weeks 1 day-40 weeks' gestation), and late term (delivered at >40 weeks' gestation). Five weeks before delivery the mean (+/-SEM) rate of rise in salivary estriol concentration was 0.50 +/- 0.13 ng/mL per week to 0.84 +/- 0.26 ng/mL per week in the early term group. The increase in rate for the middle term group was 0.32 +/- 0.06 ng/mL per week to 0.37 +/- 0.26 ng/mL per week, whereas in the late term group the rate of salivary estriol concentration rise was 0.37 +/- 0.03 ng/mL per week to -0.03 +/- 0.25 ng/mL per week.

These data demonstrate in normal pregnancies (1) that a direct, nonradiometric measure of salivary estriol concentration can be used to monitor the late pregnancy increase in estriol production, (2) that 35 weeks' gestation marks a positive inflection point of the onset of increased estriol production, and (3) that the late pregnancy rise in salivary estriol concentration shows distinct patterns that tend to be characteristic of the length of pregnancy. These data support the concept that the rate of increase of estriol production is related to the timing of the onset of labor.