A woman, who is told that their baby is 'large', will often express concerns about her baby 'getting stuck' in a sense. In reality this is rarely the case. Reasons why a baby does not easily negotiate the birth canal efficiently, usually relate to many other factors, not the size and weight of the baby.
This is why even normal sized babies can have the potential to be born by Caesarean, because the woman's labour did not 'progress'.
These factors can include:
The position of the baby's head
The strength of the contractions
Being given enough time
How the woman is feeling
Unusual shaped pelvis
The position of the baby's head
If the baby's head does not flex (having their head tucked in, looking down towards their chest) during the labour but remains 'deflexed' (the baby's head extends, looking in a forward direction), then the width of their head is increased. Sometimes the head width will increase to the point where it is larger than the mother's pelvis.
Most babies will normally have a deflexed head before the labour starts. The pressure exerted from strong uterine contractions in labour, is designed to encourage the baby to tuck their head in (or flex it), as they negotiate the birth canal. Occasionally the baby's head remains deflexed, slowing or preventing the progress of the woman's labour. A deflexed head is unlikely to repeat itself with a subsequent baby. Stronger contractions in a subsequent labour will usually help the baby to flex their head into a better position.
A baby having a deflexed head is one of the main reasons a labour does not progress efficiently. This is often given the medical term 'cephalo-pelvic disproportion' (or CPD), which means the baby's head is 'larger than the woman's pelvis'. Most caregivers will explain this variation in terms of the baby being 'too big' to fit through the woman's pelvis, rather than the baby's head being in an unusual position, even when the baby is born a relatively normal size.