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Your Baby In The Birth Canal

Normal vaginal delivery

During labor and delivery, your baby passes through your pelvic bones to reach the vaginal opening. Certain body positions give the baby a smaller shape, which makes it easier for your baby to get through this tight passage. Head facing down and the body facing toward the mother’s back is the best position for the baby to pass through the pelvis. This position is called occiput anterior.

It will be exciting to know how your baby moves down with different positions. While you are going through labor, your doctor will use the following medical terms to describe your baby’s position and movement through the birth canal.

Fetal Station

Fetal station refers to the location of the presenting part is in your pelvis.

The Presenting Part:  The presenting part is the part of the baby that leads the way through the birth canal. Most often, it is the baby’s head, but it can be a shoulder, the buttocks, or the feet.

Ischial Spines:  These are bone points on the mother’s pelvis. Normally the ischial spines are the narrowest part of the pelvis.

0 Station:  This is when the baby’s head is even with the ischial spines. The baby is said to be “engaged” when the largest part of the head has entered the pelvis.

Negative Station:  If the presenting part lies above the ischial spines, the station is reported as a negative number from -1 to -5.

Fetal Lie

This describes how the baby’s spine lines up with your spine. Your baby’s spine is location between his/her head and tailbone. Most often, your baby will settle into a position in the pelvis before labor begins.

Longitudinal Lie:  If your baby’s spine runs parallel (in the same direction) to your spine, the baby is said to be in a longitudinal lie. Nearly all babies are in a longitudinal lie.

Transverse Lie:  If the baby is sideways (at a 90-degree angle to your spine), the baby is said to be in a transverse lie.

Fetal Attitude

The fetal attitude refers to the position of the parts of your baby’s body. The normal fetal attitude is commonly called the fetal position.

In normal fetal attitude the baby’s:
  • Head is tucked down to the chest.
  • Arms and legs are drawn in towards the center of the chest.
In abnormal fetal attitudes, the baby’s:
  • Head is tilted back, so the brow or the face presents first.
  • Other body parts may be positioned behind the back. When this happens, the presenting part will be larger as it passes through the pelvis. This makes delivery more difficult.

Delivery Presentation

Delivery presentation refers to the position the baby takes to come down the birth canal for delivery.

Cephalic Presentation:  This is the best presentation. This refers to head-down presentation of your baby inside your uterus at the time of delivery. This presentation makes it easier and safer for your baby to pass through the birth canal. Cephalic presentation occurs in about 97% of deliveries. There are different types of cephalic presentation, which depend on the position of the baby’s limbs and head (fetal attitude). If your baby is in any position other than head down (cephalic presentation), your doctor may recommend a cesarean delivery.

Breech Presentation:  Breech presentation is when the baby’s bottom is down. Breech presentation occurs about 3% of the time. There are different types of breech presentation. The following are different types of breech presentation:

  • Complete Breech Presentation: A complete breech is when the buttocks present first and both the hips and knees are flexed.
  • Frank Breech Presentation: A frank breech is when the hips are flexed so the legs are straight and completely drawn up toward the chest.
  • Other Breech Presentation:These presentations occur when either the feet or knees present 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 when you deliver before your due date, or have twins or triplets.

Cardinal Movements Of Labor

Your baby will change positions as he/she passes through the birth canal. These changes are needed for your baby to fit and move through your pelvis. These movements of your baby’s head are called cardinal movements of labor. The following are different cardinal movements of labor:

Engagement:  It is called engagement when the widest part of your baby’s head has entered the pelvis. Engagement tells your doctor that your pelvis is large enough to allow the baby’s head to move down or descent.

Descent:  This occurs when your baby’s head moves down (descends) further through your pelvis. Most often, descent occurs during labor, either as the cervix dilates or after you begin pushing.

Flexion:  This occurs when the baby’s head is flexed down during descent, so that the chin touches the chest. With the chin tucked, it is easier for the baby’s head to pass through the pelvis.

Internal Rotation:  As your baby’s head descends further, the head will most often rotate so the back of the head is just below your pubic bone. This helps the head fit the shape of your pelvis. Usually, the baby will be face down toward your spine. Sometimes, the baby will rotate so it faces up toward the pubic bone. As your baby’s head rotates, extends, or flexes during labor, the body will stay in position with one shoulder down toward your spine and one shoulder up toward your belly.

Extension:  As your baby reaches the opening of the vagina, usually the back of the head is in contact with your pubic bone. At this point, the birth canal curves upward, and the baby’s head must extend back. It rotates under and around the pubic bone.

External Rotation:  As the baby’s head is delivered, it will rotate a quarter turn to be in line with the body.

Expulsion:  After the head is delivered, the top shoulder is delivered under the pubic bone. After the shoulder, the rest of the body is usually delivered without a problem.