Labour does not begin from a standing start. The weeks before it involve a set of gradual changes, most of which are not felt as anything definite.
The cervix changes before it opens
The cervix spends pregnancy as a firm closed structure. Before labour it softens, shortens and moves forward, a set of changes clinicians describe as ripening.
These changes are driven by remodelling of connective tissue rather than by contractions, and they can proceed over weeks without any noticeable sensation.
This is why two people can be equally far along and be in very different states of readiness, and why induction methods often begin by addressing the cervix.
The uterus practises coordination
Braxton Hicks contractions occur throughout pregnancy and become more noticeable late on. They are irregular, usually painless and do not progress.
Their function appears to include improving coordination between muscle fibres, so that contractions in labour act together rather than in isolated patches.
The distinction from labour lies in pattern rather than strength. Practice contractions do not become progressively closer, longer and stronger over time.
The baby settles into position
Engagement describes the baby's head descending into the pelvis. It commonly happens in the weeks before labour in a first pregnancy and often not until labour in later ones.
The descent reduces pressure under the ribs, which sometimes eases breathlessness and heartburn, while increasing pressure on the bladder and pelvis.
Position matters beyond height. How the baby is rotated affects the mechanics of labour, which is one of the things antenatal examinations assess.
Hormonal signalling shifts the balance
Through pregnancy, signalling favours keeping the uterus quiet. Toward the end, the balance shifts toward responsiveness, partly through an increase in receptors for the hormones that drive contraction.
Signals from both the fetus and the placenta contribute to this shift, which is one reason the timing of labour is not determined by the mother alone.
The onset is therefore a threshold crossed rather than a switch thrown, which is part of why prediction remains poor even at the end of pregnancy.
The signs are indicative rather than definitive
A show, mild backache, loose stools or a sudden burst of activity are all commonly reported beforehand. None of them establishes that labour will begin within a given period.
Waters breaking is more definite but does not reliably indicate timing, and management of that situation varies by circumstance and local practice.
Reduced fetal movement, bleeding, or waters breaking before term should prompt immediate contact with a maternity unit rather than waiting to see what develops.