Why Does Labour Begin?

The onset of labour is related to complex interactions of many factors. Just before birth, the muscles of the uterus contract rhythmically and forcefully. Both placental and ovarian hormones seem to play a role in these contractions. Since progesterone inhibits uterine contractions, labour cannot take place until its effects are diminished. At the end of gestation, progesterone levels falls, the level of estrogens in the mother's blood is sufficient to overcome the inhibiting effects of progesterone, and labour commences. It has been suggested that cortisol released by the fetus overcomes the inhibiting effects of progesterone so that estrogens can exert their effect. Oxytocin stimulates uterine contractions, and Relaxin assist by relaxing the pubic symphysis and helps dilate the cervix.

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