Definitions
Fetal lie
The fetus lies within the uterus and is defined in terms of its relationship to maternal anatomical parts. The lie refers to the longitudinal axis of the fetus in relation to the longitudinal axis of the mother.
With a longitudinal lie, either the fetal head or breech will be presenting. With a transverse lie, the longitudinal axis of the fetus is perpendicular to the longitudinal axis of the mother. The fetal shoulder or back present or there may be an arm, cord, or placenta presenting. An oblique lie is oriented diagonally and will usually revert to a longitudinal or transverse lie as labour ensues.Fetal presentation
The presentation refers to the part of the fetus that enters the pelvis first. A cephalic presentation is where the fetal head enters the pelvis first and when the chin is flexed onto the chest it is referred to as a vertex presentation, with the occiput as the leading point descending within the pelvis. The attitude refers to the degree of flexion of the fetal head in relation to its spine, and in normal labour a vertex presentation will be well flexed. Asynclitism refers to an oblique tilt of the vertex with the anterior or posterior parietal bone preceding the sagittal suture. In normal labour there will be minimal asynclitism.
Fetal position
The fetal position is defined as the relationship between the denominator of the presenting part and the maternal pubic symphysis, which is anterior. The denominator is the part of the fetus that defines this relationship and is a fixed bony part. The fetal position changes during the course of labour and the final position at the pelvic outlet determines the relative ease of delivery. The denominator for a vertex presentation is the occiput and in normal labour the optimal position at the pelvic outlet will be occipitoanterior (OA).