Definitions
| Term | Meaning | Examples |
|---|---|---|
| Lie | Relation of the long axis of the fetus to the long axis of the uterus | Longitudinal, transverse, oblique |
| Presentation | The part of the fetus lying over the pelvic inlet | Cephalic (vertex, brow, face), breech, shoulder |
| Presenting part | The lowest part felt on vaginal examination | Vertex, sacrum, shoulder, cord |
| Attitude | Relation of the fetal parts to each other | Flexed, deflexed, extended |
| Denominator | The fixed point on the presenting part used to name the position | Occiput, sacrum, mentum, acromion |
| Position | Where the denominator points in the maternal pelvis | LOA, ROP, LSA, RMA |
| Engagement | Widest presenting diameter has passed through the pelvic brim | 2/5 or less palpable abdominally |
| Station | Level of the leading bony point relative to the ischial spines | −3 to +3 cm |
Presentation and denominator
| Presentation | Attitude | Denominator | Presenting diameter | Length |
|---|---|---|---|---|
| Vertex | Well flexed | Occiput | Suboccipitobregmatic | 9.5 cm |
| Vertex | Deflexed (often occipitoposterior) | Occiput | Occipitofrontal | 11.5 cm |
| Brow | Partly extended | Frontal bones | Mentovertical | 13 cm |
| Face | Fully extended | Mentum (chin) | Submentobregmatic | 9.5 cm |
| Breech | Flexed or extended legs | Sacrum | Bitrochanteric | 10 cm |
| Shoulder (transverse lie) | — | Acromion | Undeliverable vaginally at term | — |
Positions of the vertex
| Position | Frequency and course |
|---|---|
| Occipitotransverse (LOT, ROT) | Commonest position at engagement |
| Occipitoanterior (LOA, ROA) | Normal; rotates 45° to direct OA |
| Occipitoposterior (LOP, ROP) | About 10–20% in early labour; most rotate forward, some arrest or deliver face to pubis |
| Direct OA or OP | At or near the outlet |
Types of breech
| Type | Legs | Notes |
|---|---|---|
| Frank (extended) | Hips flexed, knees extended | Commonest, especially in primigravidas |
| Complete (flexed) | Hips and knees flexed | Feet beside the buttocks |
| Footling or incomplete | One or both hips extended, foot presents | Highest risk of cord prolapse |
The pelvis in numbers
| Plane | Anteroposterior | Transverse | Notes |
|---|---|---|---|
| Inlet (brim) | 11 cm (obstetric conjugate) | 13 cm | Head enters transverse |
| Midcavity | 12 cm | 12 cm | Roomiest plane |
| Outlet | 13 cm (coccyx moves back) | 11 cm (intertuberous) | Head leaves anteroposterior |
The head enters in the transverse diameter, rotates in the midcavity, and leaves in the anteroposterior diameter — the basis of the mechanism of labour.
Pelvic shapes (Caldwell–Moloy)
| Type | Inlet | Clinical significance |
|---|---|---|
| Gynaecoid | Round | Most favourable; commonest in women |
| Android | Heart-shaped | Deep transverse arrest, occipitoposterior positions |
| Anthropoid | Long anteroposterior oval | Direct occipitoposterior delivery |
| Platypelloid | Wide, flattened | Engagement in the transverse, difficult descent |
Examiner questions
References
- Cunningham FG, Leveno KJ, Dashe JS, Hoffman BL, Spong CY, Casey BM, eds. Williams Obstetrics. 26th ed. New York: McGraw Hill; 2022.