Delivery presentation describes the way the fetus is positioned to come down the birth canal for delivery. The presentation is defined by fetal attitude, the presenting part, and maternal and fetal landmarks.
THE DELIVERY PROCESS
The delivery process is described in terms of fetal station, lie, attitude, and presentation.
This is the relationship between the presenting part, whether that be the head, shoulder, buttocks, or feet, and two parts of the maternal pelvis called the ischial spines. Normally the ischial spines are the narrowest part of the pelvis, and are a natural measuring point for the delivery progress. If the presenting part lies above the ischial spines, the station is reported as a negative number from 0 to -3 where each number is a centimeter. If the presenting part lies below the ischial spines, the station is reported as a positive number from 0 to 3 where each number is a centimeter. The baby is said to be "engaged" in the pelvis when it reaches 0 station.
This is the relationship between the head to tailbone axis of the fetus and the head to tailbone axis of the mother. If the two are parallel, then the fetus is said to be in a longitudinal lie. If the two are at 90-degree angles to each other, the fetus is said to be in a transverse lie. Nearly all (99.5%) fetuses are in a longitudinal lie.
The fetal attitude describes the relationship of the fetus' body parts to one another. The normal fetal attitude is commonly referred to as the fetal position: the head is tucked down to the chest, with arms and legs drawn in towards the center of the chest. Abnormal fetal attitudes may include a head that is extended back or other body parts extended or positioned behind the back. Abnormal fetal attitudes can increase the diameter of the presenting part as it passes through the pelvis, increasing the difficulty of birth.
Cephalic (head-first) presentation:
Cephalic presentation is considered normal and occurs in about 97% of deliveries. There are different types of cephalic presentation, which depend on the fetal attitude. If the fetus' head is extended back, then the chin, face, or forehead will exit first depending on the degree of extension. This is not considered preferable since this part of the fetal head is not the smallest, thus increasing the difficulty of the birth. Preferably the fetal head is flexed down to the chest resulting in a "vertex" delivery, in which the crown of the head exits first.
A cesarean delivery may be done for any fetal position other than cephalic.
Breech presentation is considered abnormal and occurs about 3% of the time. A complete breech birth occurs when the buttocks present first, and both the hips and the knees are flexed. A frank breech occurs when the hips are flexed so the legs are completely drawn up towards the chest. Other breech positions occur when either the feet or knees come first.
The shoulder, arm, or trunk may exit first if the fetus is in a transverse lie. This type of birth occurs less than 1% of the time. Transverse lie is more common with premature delivery or multiple gestations.