Factors Influencing the Process of Labor and Birth

Core Concepts

The Passenger: the fetus

The fetus is described by several characteristics that together predict how well it will fit through the pelvis:

• Lie — the relationship of the fetal long axis to the maternal long axis: longitudinal (head or buttocks down), transverse (sideways), or oblique.
• Presentation — the fetal part that enters the pelvis first: cephalic (head), breech (buttocks or feet), or shoulder.
• Position — the relationship of a reference point on the presenting part to the maternal pelvis, such as occiput anterior (baby facing the maternal back). Occiput anterior positions are generally the most favorable for vaginal birth.
• Attitude — the degree of flexion of the fetal head. A well-flexed head presents its smallest diameter to the pelvis, which is why "tuck the chin" matters.
• Station — the level of the presenting part in relation to the ischial spines, measured in centimeters above or below zero. Zero station means the presenting part is at the spines; engagement is typically defined as reaching zero station.
• Fetal size — a larger passenger needs a passageway that can accommodate it; size is estimated by the provider, not by the nurse.

The Passageway: the pelvis and soft tissues

The passageway has a bony part and a soft part. The bony pelvis is classically described by four shapes — gynecoid (the most common and generally most accommodating), android, anthropoid, and platypelloid — and by its diameters, which the fetus must negotiate during descent. The soft passageway includes the cervix (which must efface and dilate), the vagina, and the pelvic floor muscles, which relax and stretch under the influence of pregnancy hormones. A passageway that is adequate in shape can still slow labor if the soft tissues are tense or if the person is in a position that narrows the pelvic outlet.

The Powers: contractions and pushing

The powers are the forces that move the passenger through the passageway. Primary powers are the involuntary uterine contractions: their frequency, duration, and intensity build as labor progresses, and their job is to produce effacement and dilation of the cervix. Secondary powers are the voluntary bearing-down efforts of the pregnant person, which typically join in once the cervix is fully dilated. A key nursing concept: progress is measured by cervical change and fetal descent, not by the clock — contractions of any pattern are only "effective" if they produce change.

Position: the pregnant person's position and mobility

The position of the pregnant person influences both the powers and the passageway. Upright positions — standing, walking, sitting, squatting, kneeling — use gravity to aid descent and keep the pelvic outlet open, and they are generally associated with more effective contractions and shorter labor. Supine (flat on the back) positions can compress the aorta and vena cava (aortocaval compression), which can lower cardiac output and placental blood flow and make contractions feel weaker. Positioning is a nursing intervention: the nurse offers positions and helps the person change them, always respecting the person's preference and energy level.

Psyche: the psychological dimension

The pregnant person's emotional state is the fifth P. Fear raises muscle tension, and tension can amplify pain, which in turn raises fear — the classic fear-tension-pain cycle described in childbirth education. Stress and anxiety also influence hormones that interact with the hormones driving contractions, which is why a calm, supported, informed person often labors more effectively than one who is frightened and alone. The nurse's presence, explanation, and encouragement are direct interventions on this factor.

The Placenta: the sixth P

Some frameworks add the placenta because it sits between the passenger and the oxygen supply. Its position (for example, anterior versus posterior) can affect how easily fetal heart tones are heard, and its function determines how well the fetus tolerates the brief reductions in blood flow that occur during contractions. Placental problems are covered in the complications chapters; here the point is that fetal well-being during labor depends on placental exchange working continuously.

Common Confusions