The phrase "spontaneous labour" means a situation where a pregnant woman goes into labor naturally, without the need for any medical intervention. Full definition
Most pregnancies go to full term, which means that a woman will go into spontaneous labour some time between the 37 and 42 week mark. (aimsireland.ie)
A Cochrane review comparing induction of labour before 40 weeks for a suspected big baby with waiting for spontaneous labour, found that induction decreased the incidence of shoulder dystocia from 6.8 % to 4.1 %. (midwifethinking.com)
It is still a mystery as to what exactly occurs to initiate spontaneous labour with some theories saying it is due to the release of oxytocin in the pregnant woman while others say that it occurs when the fetus secretes a protein from its lungs. (aimsireland.ie)