Amniotic fluid volume and onset of labor in physiological pregnancy

We measure, by means of ultrasound, the amniotic fluid volume (expressed as maximal vertical pocket or MVP) in 646 normal pregnancies at the 39th gest ational week. Our aim is to evaluate the possible correlation between MVP a nd onset of the labor. In a 2-week follow-up, the onset of the labor is con sidered the "event" variable in a time-dependent statistical analysis. Univ ariate analysis (Kaplan-Meier algorithm) describes a different trend in pre dicting the onset of labor when a stratification of MVP <50 and greater tha n or equal to 50 mm was performed (chi(2) = 7.91 p < 0.0049 with 1 df, Bres low-Gehan test). The first category was comprised of 496 fetuses with a med ian (min-max) MVP of 39 mm (25-49), the second category of 150 fetuses with a MVP of 57 mm (50-100). The results suggest that lower levels of MVP are associated to a higher percentage of the onset of labor. Furthermore, in ou r measurement, performed at the 39th gestational week, the correlation with the events is higher within the 40th gestational week. In fact, at 7 days from the amniotic fluid measurement, the onset of labor and the subsequent delivery is observed in 80.65 and 73.00% of the cases when they are stratif ied according to MVP <50 and greater than or equal to 50 mm. At the end of the follow-up, instead, the percentage of "events" is similar, 88.10 and 86 .67%, respectively. Adjustment for covariates (Cox analysis), as well as ma ternal age, neonatal weight, and obstetrics history, show an odds ratio (95 % C.I.) of 2.08 (1.61-2.69) for MVP, using the above cutoff level. In physi ological pregnancy, lower levels of amniotic fluid at term correlated to a higher probability of the onset of labor.