The mechanisms involved in the initiation of labour and uterine contraction during pregnancy are very complex, and not fully understood. During pregnancy, the uterus performs two functions, namely, uterine relaxation and contraction. For most of the pregnancy, the uterus is in a relaxed state and during labour it contracts regularly and forcibly in order to expel the foetus through the birth canal. A uterotonic (or uterotonin) is a trigger that is responsible for the transition from relaxation to the onset of labour.