When a person is in labor, healthcare providers track the baby’s progress through the pelvis toward delivery. Cervical dilation is a commonly discussed measurement, but pelvic station provides a clear, objective picture of the baby’s descent. Tracking this movement helps clinicians understand if labor is advancing efficiently and when intervention might be necessary.
What Pelvic Station Measures
Pelvic station is a numerical system describing the relationship between the lowest part of the baby, typically the head, and fixed bony points within the mother’s pelvis. The system is expressed in centimeters, ranging from negative numbers to zero and then to positive numbers.
A negative station means the baby is still positioned above the reference point in the pelvis, which is considered high or “floating.” Zero station indicates the baby is aligned with the reference point, signifying that the head is “engaged” into the pelvis. Positive numbers mean the baby has moved past the reference point and is progressing toward the vaginal opening. Each change in number, such as moving from -3 to -2, represents a descent of about one centimeter.
Identifying the Zero Point
The entire pelvic station system hinges on a specific anatomical landmark known as the ischial spines. These spines are two bony projections located inside the mother’s pelvis and represent the narrowest point of the mid-pelvis.
Healthcare providers use these spines as the fixed point to standardize the measurement of fetal descent. When the lowest point of the baby’s head is directly even with the ischial spines, the pelvic station is defined as zero. This zero point serves as the dividing line between the baby being high in the pelvis and having descended more deeply into the birth canal.
Interpreting -3 Station
A pelvic station of -3 means the lowest part of the baby’s head is positioned three centimeters above the imaginary line connecting the mother’s ischial spines. This value is part of the range of negative stations, which can run from -5 to -1, with -5 being the highest point in the pelvis. At a -3 station, the baby is still considered quite high in the pelvis and is not yet engaged.
Finding a station of -3 is common in the days or weeks leading up to labor, or during the initial, early phase of labor. This reading confirms that the head has not yet descended into the mid-pelvis and is still mobile. It does not suggest a problem with the baby or the labor process itself, but rather indicates that the journey through the pelvis has just begun.
Station Progression and Labor Timing
After an initial reading like -3, the focus of labor assessment shifts to tracking the baby’s continuous descent through the pelvis. The progression from negative stations, such as -4, -3, and -2, signifies the baby is moving into the pelvic inlet and eventually reaching the zero station, which is full engagement. This movement is often powered by the force of uterine contractions during active labor.
Once the baby reaches the zero station, the progression continues into positive numbers (+1, +2, +3, etc.), indicating descent below the ischial spines. A station of +4 or +5 means the baby’s head is nearing the vaginal opening and is ready for delivery, sometimes referred to as crowning. Healthcare providers use this progression, along with cervical dilation, to estimate the timing of labor and determine when the pushing phase should begin. Monitoring this measurement is an ongoing part of labor management.