It still is unclear what causes labour to begin, there are several theories One is that the mother's pituitary gland secretes oxytocin when the baby is fully developed and ready to be born. Oxytocin is the hormone that stimulates contractions.
For many years, it was believed that the mother's body was responsible for starting labour. Some researchers now believe that the baby actually starts labour. The baby sends a signal to the mother's body that causes labour to start. One theory is that the baby's lungs secrete an enzyme when they are fully developed. This causes prostaglandins to be released into the mother's system. The prostaglandins then trigger changes in the cervix and contractions. Another theory is that the baby's adrenal glands send a signal to start labour. When the baby is ready to be born, the adrenal glands produce hormones. These hormones cause hormonal changes in the mother. These changes are responsible for the process that starts labour.
While there are many theories, no one really knows what triggers labour. This process remains a mystery. As research continues, the answer to this question may soon be answered.
Labour may start in several ways. The signs that labour may be starting include regular contractions. Some women get a 'Show', or ‘Shows’ (can be more than one), not all women do. A 'show' is a small amount of sticky pinkish mucous, there may be some blood mixed with this. Your waters may break. Some women experience this before contractions begin, some after the contractions begin, and some do not experience the waters breaking at all (sometimes the midwife has to break the waters later in the labour). If your waters break, at any time, do contact your midwife immediately. They will want to check you and the baby. Some women feel sick or nauseous, some women experience diarrhoea.
It you are unsure if your labour has commenced do ring your local midwife or phone the midwives at your nearest maternity unit labour ward or birth centre.