Your baby in the birth canal
During labor and delivery, your baby must pass through your pelvic bones to reach the vaginal opening. The goal is to find the easiest way out. Certain body positions give the baby a smaller shape, which makes it easier for your baby to get through this tight passage.
The best position for the baby to pass through the pelvis is with the head down and the body facing toward the mother's back. This position is called occiput anterior.
Shoulder presentation; Malpresentations; Breech birth; Cephalic presentation; Fetal lie; Fetal attitude; Fetal descent; Fetal station; Cardinal movements; Labor-birth canal; Delivery-birth canal
Certain terms are used to describe your baby's position and movement through the birth canal.
Fetal station refers to where 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.
- If the presenting part lies above the ischial spines, the station is reported as a negative number from -1 to -5.
In first-time moms, the baby's head may engage by 36 weeks into the pregnancy. However, engagement may happen later in the pregnancy, or even during labor.
This refers to how the baby's spine lines up with the mother's spine. Your baby's spine is between his head and tailbone.
Your baby will most often settle into a position in the pelvis before labor begins.
- If your baby's spine runs in the same direction (parallel) as your spine, the baby is said to be in a longitudinal lie. Nearly all babies are in a longitudinal lie.
- If the baby is sideways (at a 90-degree angle to your spine), the baby is said to be in a transverse lie.
The fetal attitude describes the position of the parts of your baby's body.
The normal fetal attitude is commonly called the fetal position.
- The head is tucked down to the chest.
- The arms and legs are drawn in towards the center of the chest.
Abnormal fetal attitudes include a head that 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 describes the way the baby is positioned to come down the birth canal for delivery.
The best position for your baby inside your uterus at the time of delivery is head down. This is called cephalic presentation.
- This position 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, your doctor may recommend a cesarean delivery.
Breech presentation is when the baby's bottom is down. Breech presentation occurs about 3% of the time. There are a few types of breech:
- A complete breech is when the buttocks present first and both the hips and knees are flexed.
- A frank breech is 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 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
As your baby passes through the birth canal, the baby's head will change positions. 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.
- This is when the widest part of your baby's head has entered the pelvis.
- Engagement tells your health care provider that your pelvis is large enough to allow the baby's head to move down (descend).
- This is 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.
- During descent, the baby's head is flexed down so that the chin touches the chest.
- With the chin tucked, it is easier for the baby's head to pass through the pelvis.
- 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.
- 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.
- As the baby's head is delivered, it will rotate a quarter turn to be in line with the body.
- 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.
Kilpatrick S, Garrison E. Normal labor and delivery. In: Gabbe SG, Niebyl JR, Simpson JL, et al, eds. Obstetrics: Normal and Problem Pregnancies. 7th ed. Philadelphia, PA: Elsevier; 2017:chap 12.
Lanni SM, Gherman R, Gonik B. Malpresentations. In: Gabbe SG, Niebyl JR, Simpson JL, et al, eds. Obstetrics: Normal and Problem Pregnancies. 7th ed. Philadelphia, PA: Elsevier; 2017:chap 17.