The delivery process is described in terms of fetal station, lie, attitude, and presentation.
This is the relationship between the presenting part of the baby -- the head, shoulder, buttocks, or feet -- and two parts of the mother's pelvis called the ischial spines. Normally the ischial spines are the narrowest part of the pelvis. They 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 -1 to -5 (each number is a centimeter). If the presenting part lies below the ischial spines, the station is reported as a positive number from +1 to +5. The baby is said to be "engaged" in the pelvis when it is even with the ischial spines at 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.
Rarely, the fetus' head is extended back, and the chin, face, or forehead will present first depending on the degree of extension. This is a more difficult delivery, because this is not the smallest part of the fetus' head. It may result in a need for cesarean delivery.
A cesarean delivery may be recommended for any of the fetal positions other than cephalic.
Breech presentation is considered abnormal and occurs about 3% of the time. A complete breech presentation occurs when the buttocks present first, and both the hips and knees are flexed. A frank breech occurs when the hips are flexed so the legs are straight and completely drawn up toward the chest. Other breech positions occur when either the feet or knees come out first.
The shoulder, arm, or trunk may present first if the fetus is in a transverse lie. This type of presentation occurs less than 1% of the time. Transverse lie is more common with premature delivery or multiple pregnancies.
Lanni SM, Seeds JW. Malpresentations. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics: Normal and Problem Pregnancies. 5th ed. Philadelphia, Pa: Elsevier Churchill Livingstone, 2007;chap 17.
Cunningham FG, Leveno KJ, Bloom SL, et al. Breech presentation and delivery. In: Cunnigham FG, Leveno KL, Bloom SL, et al, eds. Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010:chap 24.
Cunningham FG, Leveno KJ, Bloom SL, et al. Prenatal diagnosis and fetal therapy. In: Cunnigham FG, Leveno KL, Bloom SL, et al, eds. Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010:chap 13.
Linda J. Vorvick, MD, Medical Director, MEDEX Northwest Division of Physician Assistant Studies, University of Washington, School of Medicine; Susan Storck, MD, FACOG, Chief, Eastside Department of Obstetrics and Gynecology, Group Health Cooperative of Puget Sound, Bellevue, Washington; Clinical Teaching Faculty, Department of Obstetrics and Gynecology, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.