NORMAL LABOUR

(1) Oestrogen theory:(I) Hormonal factors:During pregnancy, most of the oestrogens are present in a bindingform. During the last trimester, more free oestrogen appearsincreasing the excitability of the myometrium and prostaglandinssynthesis.(2) Progesterone withdrawal theory:Before labour, there is a drop in progesterone synthesis leading topredominance of the excitatory action of oestrogens.(3) Prostaglandins theory:Postaglandins E2 and F2a are powerful stimulators of uterine muscleactivity.