When the presenting part of the fetus is entirely out of the pelvis and is freely movable above the inlet, it is said to be floating (Fig. 8-1A).
When the presenting part has passed through the plane of the inlet but engagement has not occurred, it is said to be dipping (Fig. 8-1B).
By definition, engagement (Fig. 8-1C) has taken place when the widest diameter of the presenting part has passed through the inlet. In cephalic presentations, this diameter is the biparietal, between the parietal bosses. In breech presentation, it is the intertrochanteric.
The process of engagement.
In most women, once the head is engaged, the bony presenting part (not the caput succedaneum) is at or nearly at the level of the ischial spines. Radiologic studies have shown that this relationship is not constant and that in women with deep pelves, the presenting part may be as much as 1 cm above the spines even though engagement has occurred.
The presence or absence of engagement is determined by abdominal or vaginal examination. In primigravidas, engagement usually takes place 2 to 3 weeks before term. In multiparas engagement, engagement may occur any time before or after the onset of labor. Engagement tells us that the pelvic inlet is adequate. It gives no information as to the midpelvis or the outlet. Although failure of engagement in a primigravida is an indication for careful examination to rule out disproportion, abnormal presentation, or some condition blocking the birth canal, it is no cause for alarm. The occurrence of engagement in normal cases is influenced by the tonus of the uterine and abdominal muscles.
Station is the relationship of the presenting part to an imaginary line drawn between the ischial spines (Fig. 8-2). The location of the buttocks in breech presentations or the bony skull (not the caput succedaneum) in cephalic presentations at the level of the spines indicates that the station is zero. Above the spines, the station is −1, −2, and so forth, depending on how many centimeters above the spines the presenting part is. At spines −5, it is at the inlet. Below the spines, it is +1, +2, and so forth. There are various relationships between station and the progress of labor.
Station of the presenting part.
In nulliparas entering labor with the fetal head well below the spines, further descent is often delayed until the cervix is fully dilated
In nulliparas beginning labor with the head deep in the pelvis, descent beyond the spines often takes place during the first stage of labor
An unengaged head in a nullipara ...