Favourable foetal lie, presentation and position for vaginal birth
Vaginal birth is easiest when the baby lies in line with the mother, head first, facing her spine, with the chin tucked onto the chest.
Remember
Vaginal birth requires a longitudinal lie, in which the baby's long axis runs along the mother's. It is easiest when the baby is head first, with the occiput (the bony back of the skull) at the front of the pelvis and the chin tucked onto the chest.
Memory aid
the lie is the line the baby makes
Picture the baby's line running the same way as the mother's line: longitudinal means along and transverse means across.
Three linked terms describe how a baby sits in the uterus. The lie is the angle between the baby's long axis (head to bottom) and the mother's long axis. In a longitudinal lie the baby's long axis is aligned with the mother's. In an oblique lie the baby's axis slants across the mother's. In a transverse lie the baby is placed across the uterus. The presentation is the part of the baby that leads, such as the head, the bottom or a shoulder. The position is the way the baby faces within that presentation.
Vaginal birth needs a longitudinal lie, because only then can the head or the bottom move into the pelvis first. Head-first babies are called cephalic and bottom-first babies are breech, and both are presentations within a longitudinal lie. A shoulder presentation should not be attempted vaginally.
In a head-first baby, the position is named from the occiput, the bony back of the skull, unless the chin leads. When the occiput lies at the front of the mother's pelvis, the baby faces her spine and is occipitoanterior, the easiest position for a vaginal birth. In the occipitoposterior position, called 'back to back', the occiput is towards the mother's spine and the face looks forwards. A head lying sideways in the pelvis is occipitotransverse, and many babies in that position rotate to occipitoanterior as labour goes on.
The attitude is how far the neck is flexed (chin towards chest). A well-flexed head presents a smaller diameter, the span across the part of the head that leads through the pelvis. The ideal combination is an occipitoanterior position with the chin tucked onto the chest. An extended neck or an occipitoposterior position is linked with a larger leading diameter and can slow the first or second stage of labour.
Key facts
- Lie
- the direction of the baby's long axis compared with the mother's, which can be longitudinal, oblique or transverse.
- Vaginal birth
- needs a longitudinal lie, with either the head or the bottom entering the pelvis first.
- Easiest position
- occipitoanterior, although the other positions can still deliver.
- Back to back
- the occipitoposterior position, with the occiput towards the mother's spine and the baby's face looking forwards.
- Neck flexion
- more flexion (chin on chest) gives a smaller head diameter, whereas poor flexion gives a larger diameter and can slow descent.
- Breech
- bottom first, and vaginal birth is possible but rarer.
Exam traps
- A head lying sideways in the pelvis (occipitotransverse) is not abnormal, but even with good neck flexion it has not reached the best alignment, which comes only after rotation to occipitoanterior.
- Only slight neck flexion leaves a larger head diameter leading than a chin tucked onto the chest, even in an occipitoanterior position.
- Tucking the chin onto the chest helps, but an occipitoposterior position is itself linked with a larger presenting diameter and slower labour, so good flexion does not fully make up for it.
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