Anatomy and Physiology of Childbirth

Anatomy and Physiology of Childbirth

Key Points to Cover

• The Process of Labor:

  • The anatomical passageway

  • The fetal contribution to labor

  • The effect of maternal position on labor

  • The uterine contraction

  • The psychologic contribution to labor

The Labor Process

• Labor is defined as a coordinated sequence of contractions, which leads to:

  • Cervical dilatation/effacement

  • Fetal descent through the birth canal

The 5 P's of Labor

• This model coordinates the factors affecting the birth process:

  1. Passageway

  2. Passenger

  3. Position

  4. Powers

  5. Psyche

Passageway

Composed of two components:

• Bony Pelvis (diameters and planes)

  • Inlet

  • Mid-pelvis - spans between the ischial spines

  • Outlet

• Soft Tissues

  • Cervix

  • Vagina

  • Perineum

• Goal: Facilitate the largest part of the baby to pass through the smallest part of the pelvis.

Pelvic Anatomy

• False pelvis

• True pelvis: Divided into different areas for understanding measurements

  • Diagonal conjugate: Extends from the lower border of the symphysis pubis to the sacral promontory.

• Clinical measurements include:

  • True conjugate, approx. $10 ext{ cm}$

  • Diagonal conjugate, approx. $12 ext{ cm}$

  • Obstetrical conjugate, approx. $11 ext{ cm}$

Measurement of Pelvic Planes

• Figures: Detailed illustrations of pelvic structures and measurement points, such as the sacral promontory and transverse diameter.

Types of Pelvic Shapes

• Gynecoid

• Anthropoid

• Android

• Platypelloid

Cervical Measurement

• Gauging cervical dilation:

  • Before labor: Cervix is long approx. 2.5 cm, sides feel thick, canal closed.

  • During labor: Dilation progresses from 1 cm to 10 cm.

The Passenger (Fetal Contribution)

• Contributes to labor by:

  • Tolerance of contractions

  • LAPPS Model:

  • Lie: Position of the fetus in relation to the maternal spine

  • Attitude: Flexed or extended positions of the fetal body

  • Presentation: The part of the fetus that presents to the cervix (e.g. occiput, mentum)

  • Position: Relationship of the fetal presenting part to the maternal pelvis

  • Station: The fetal head's position in relation to the ischial spines, marked in centimeters (e.g., $-5$ to $+5$).

Fetal Lie Types

• Longitudinal Lie: The long axis of the fetus is parallel to the mother's long axis.

• Transverse Lie: The long axis of the fetus is at right angles to that of the mother.

Fetal Attitude

• Normal attitude: Flexion of limbs against trunk

• Abnormal attitude: Extension occurs.

Fetal Skull Anatomy

• Components:

  • Occipital bone, Parietal bones, Frontal bone

  • Fontanelles: Anterior (bregma) and posterior

• Measurements of the fetal skull:

  • Suboccipitobregmatic diameter: Approx. $9.5 ext{ cm}$

  • Occipitofrontal diameter: Approx. $11.75 ext{ cm}$

  • Occipitomental diameter: Approx. $13.5 ext{ cm}$

Presentation Types

• Cephalic Presentations: Normal vertex presentation

• Breech Presentations: Baby is positioned with buttocks or feet down, which can complicate labor and delivery.

  • Frank breech: Most common

  • Complete breech

  • Footling breech: May be single or double.

Fetal Positioning within the Maternal Pelvis

• Four quadrants for positioning:

  1. Right anterior (RA): Fetus is positioned with the back facing the mother's right side and the head down, often leading to easier labor progress.

  2. Left anterior (LA): Fetus is positioned with the back facing the mother's left side and the head down, which can facilitate optimal fetal descent during labor.

  3. Right posterior (RP): Fetus is positioned with the back facing the mother's right side and the head down, sometimes causing discomfort for the mother and leading to longer labor durations.

  4. Left posterior (LP): Fetus is positioned with the back facing the mother's left side and the head down, which may result in a more challenging labor due to potential back pain for the mother and slower progress in the delivery process.

Station

• Depicts fetal descent marked in centimeters:

  • High head: $-4$

  • Engaged: $0$ (at ischial spines)

  • Deeply engaged: $+2$

Cardinal Movements of Labor

• A series of maneuvers the fetus undertakes to leave the birth canal:

  1. Descent: Head enters inlet in the occipito-transverse position.

  2. Flexion: Head flexes to chest.

  3. Internal Rotation: Head rotates from OT to OA.

  4. Extension: Head passes under the pubic bone.

  5. Restitution: After the birth of the head, it turns to realign with shoulders.

  6. External Rotation: Shoulders rotate to an antero-posterior position.

  7. Expulsion: Birth of the remainder of the body, anterior shoulder comes out first.

Observing Cardinal Movements

• ET: Extension during crowning — The vertex is crowning and the head extends under the pubic bone.

• R: Restitution beginning — Head is delivered in OA position and is beginning to turn to LOT.

• E: External rotation preparation for expulsion — Head is LOT and shoulders are turning anteroposteriorly.

Maternal Position Impact

• Importance of Position:

  • Let gravity assist

  • Avoid supine/low Fowler's positions (no flat on back)

  • Position influences:

  • Contractions

  • Labor progression

  • Fetus' position

  • Comfort and satisfaction of the mother

• Upright/Lateral Positions: Enhance blood flow to fetus and assist head rotation.

Powers of Labor

• Contractions:

  • Primary powers: Responsible for cervical effacement, dilation, descent and rotation of the fetus, and separation and expulsion of the placenta.

  • Hydrostatic Force: The pressure of uterine contractions on fluid and membranes increase the pressure on the cervix, aiding in dilation and effacement.

  • Secondary powers: Voluntary abdominal efforts during labor.

Theories of Labor Onset

• Mechanisms contributing to onset include:

  • Hormonal Changes:

  • Withdrawal of progesterone (suppresses uterine irritability)

  • Increased oxytocin and prostaglandin

  • Increased endothelin production (potent vasoconstrictor)

  • Increased estrogen (promotes oxytocin production and formation of estrogen receptors in the myometrium)

  • Role of Fetal Cortisol: Promotes labor initiation.

Contraction Cycle

• Described in terms of:

  • Increment

  • Peak

  • Decrement

  • Intensity: Mild to strong

  • Duration: Seconds

  • Frequency: Minutes and fractions of a minute

Psychosocial Factors Affecting Labor

• Factors of influence include:

  • Cultural: Beliefs and practices

  • Support system: Family, friends, healthcare providers

  • Personal experiences: Previous birth experience, trauma

  • Knowledge and confidence: Impact on fear and pain perception

Labor Pain Cycle

• Cycle illustrated as a feedback loop:

  • Pain increases fear, tension, and anxiety, stimulating catecholamines.

  • Increased catecholamines lead to decreased contraction efficiency.

Maternal Psychological Adaptation during Labor

• Stages of Adaptation:

  1. Anticipation: Positive outlook

  2. Inward focus: Quiet and self-reflective

  3. Increased distress: Experiencing loss of control and irritability

Role of Nursing Staff

• Nurses have a profound impact on maternal positioning and psychologic support during labor, influencing overall outcomes and satisfaction.