The million dollar question! But one that many women wonder. Labour can start on its own (spontaneously) or due to methods called induction, where the labour is started artificially before it has begun on its own.
Nobody really knows why labour starts, but we think it’s the baby who actually starts labour. It is thought that the baby releases stress chemicals that kick-start a hormonal process in the mother that starts labour.
Normal human labour starts any time between 37 and 42 weeks of pregnancy; in my practice, I find that most women go into labour between 38 and 41 weeks, and I have yet to care for a woman whose pregnancy extends beyond 10 days past 40 weeks.
If a baby is born before 37 weeks, the baby would be considered to be preterm, and if the baby was born after 42 weeks, it would be post-term.
The trouble is that we give women “due dates” that equate to 40 weeks of pregnancy, and maybe we aren’t so good at explaining that it’s an estimated due date, not an exact due date.
In my practice, I find that labour usually starts when women feel relaxed and comfortable, when the baby is well-positioned, when women feel well-prepared for labour, and when all the last-minute things have been taken care of. Labour seldom starts when visitors are planned to arrive (or leave), or before a major event, or before the woman feels ready to meet her baby.
I find that women who feel well-prepared, confident about birth and ready to meet their babies, tend to go into labour and have a quick, easy birth.