Saliva oestriol, oestradiol, oestrone and progesterone levels in pregnancy: spontaneous labour at term is preceded by a rise in the saliva oestriol:progesterone ratio.

Saliva steroid levels reflect the unbound unconjugated (free, biologically active) plasma hormone levels. Saliva oestriol (E3), oestradiol (E2), oestrone (E1) and progesterone levels were estimated by radioimmunoassay in saliva samples obtained twice a week from 18 weeks gestation until 38 days before delivery and then daily until the spontaneous onset of labour at term from 20 normal pregnant women. The overall percentage increases in the median concentrations of E3, E2, E1 and progesterone were 718, 370, 80 and 214%, respectively, in the last 20 weeks and 149, 82, 24 and 41%, respectively, in the last 6 weeks of pregnancy. The median E3:progesterone ratio rose slowly from 0.65 at 20 weeks before delivery to 1.0 at 5 weeks before delivery and then rapidly to 1.65 one day before the spontaneous onset of labour. There was an increase in the E3:progesterone ratio from less than 1 to greater than 1 before labour in every subject.