Abdominal Function in Birth 101

There are many changes that take place during pregnancy. One of the most noticeable changes is your growing belly.

As your baby and uterus grows the abdominal wall expands. With this expansion the abdominal muscles lengthen. With lengthening there is loss of function.

How does this influence birth?

The simple answer … baby position and baby expulsion.

To understand how the abdominal wall influences baby positioning and expulsion first you need to understand the abdominal muscles anatomy and function.

Abdominal Muscle Anatomy 101

There are four layers of abdominal muscles (starting from the most superficial to deep): rectus abdominis (RA), external (EO) and internal obliques (IO), transverse abdominus (TvA).

The midline of the abdominal muscles is a connective tissue called the linea alba.

The abdominal muscles run from the anterior or side ribs down to the pelvis in either a vertical, diagonal or horizontal direction.

Abdominal Muscle Function 101

The three main functions of the abdominal wall is to hold the abdominal contents (your organs or insides) in place, stability and movement of your spine and pelvis.

Each abdominal muscle has a slightly different role for these functions. For example the RA flexes/bends your spine or tips your pelvis back and can increase your intra-abdominal pressure or force an exhale. The obliques side bend and twist/rotate your spine, but can also tip the pelvis back (EO), force exhalation (IO) and compress the abdominal organs into the diaphragm (IO). Where as the TvA stabilizes the spine and pelvis prior to upper or lower limb movement.

The muscles will perform these functions in a balanced way when in a healthy state, and may exaggerate these functions when in an unhealthy state. For example, when the IO is overactive or tight is narrows the waist and pushes your abdominal organs into the diaphragm, which can cause digestion issues, difficulty breathing and pain. This can in turn make the RA pull a part causing a loss of tissue integrity in the linea alba and increased intraabdominal pressure.

Pregnancy Abs 101

Now that you have a more clear understanding of the abdominal muscles, we can apply this to pregnancy and birth. As baby grows the uterus becomes an abdominal organ and thus needs to be supported by these muscles.

As I stated above the abdominal muscles lengthen during this process. The way a muscle works is by the fibers sliding across each other, like if you weaved your fingers together and slide them side by side. When a muscle contracts the fibers slide tighter together, when the muscles is stretched the fibers slide further apart. When the muscle is in a balanced relaxed state the fibers are in what I like to call the “Goldy locks” position, ready for proper functioning.

We know that a prolonged stretched muscle will create more contractile fibers to keep up with the demand placed on the muscle. And even though this has never been studied with pregnant humans, it is something that likely occurs to maintain some abdominal functioning.

We also know that the RA stretches 115% in length by 38 weeks gestation, which overloads the length tension (or Goldy Locks state) relationship of the fibers disrupting the capacity to produce normal tension with contraction. However, the body makes up for this by creating more contractile fibers. So you’re body is still capable of producing enough power to hold your abdominal contents in place, and stabilize or move your spine and pelvis, if the muscles are in a healthy state.

Another change that occurs in pregnancy is a widening and thinning of the linea alba, which is the midline between the two RA halves. This is the area when under uncontrolled intraabdominal pressure you see a doming (or gap postpartum), and when it becomes dysfunctional or a certain separation, it’s considered a diastasis recti abdominus (DRA). The combination of the DRA or abdominal separation, uncontrolled intrabdominal pressure and overactive or tense muscles alters the available space in the trunk.

Baby Position

There are two ways the abdominal wall can effect baby positioning.

The first is the integrity of the muscles strength and endurance for posture. It is very common to slouch and tuck the pelvis under when sitting and can effectively encourage baby to settle into an occiput posterior (or “sunny side up” with the face anterior) position. With a compromised abdominal wall the body has a harder time staying in good posture. Thus finding strategies to help cope with this like putting pillows behind your back and setting reminders to sit up on your sitz bones will help, in addition to proper core activation with daily mindfulness and exercise.

The second way the abdominal wall effects baby positioning is by altering the shape of the trunk. This happens when an abdominal muscle, usually the obliques are tight or overactive and “squeeze” your waist, leading to a weakening along the linea alba and ultimately a DRA. This is like wearing a tight skirt that restricts your leg movement. So baby will move into spaces that are less restrictive, which is usually bulging forward over the pelvis, also known as a pendulous uterus. This alters the ideal angle of axis for the baby to descend into the pelvis for birth and is the leading cause of asynclitic (tilted) heads. So even though baby’s who are not in ideal position can be born, there is evidence to suggest that when they have optimal support from the abdominal wall it is easier for them to get into good head position (being flexed neck, tucked chin, facing back).

Baby Expulsion

Yes, baby positioning can be important for birth, but so it the ability to get baby out, for which the abdominal wall plays a roll. The uterus is a muscle that contracts to push baby out. But it is helped by the abdominal muscles.

There are a few ways the abdominal wall plays a role in the expulsion phase (or second/pushing phase of labor). The first is related to the angle of axis mentioned above. When the integrity of the abdominal wall is compromised the efforts of each uterine contraction is placed in a biomechanical disadvantage. Meaning instead of with each contraction the intraabdominal pressure pushes the baby down through the pelvis, it actually causes the uterus to push forward into the abdominal wall. This is a problem.

The second is the amount of power necessary to expel baby. Yes, the uterus is a powerful organ that is primed to labor and push a baby through the pelvis. However it has it’s limits and requires assistance from the abdominal wall. As I mentioned before the rectus abdominus increases the intra-abdominal pressure with contraction. And that is exactly what is needed to push baby out. So the extra voluntary contraction of the abdominal wall, especially the RA, joins the uterine contractions to provide extra power to push. This will be compromised if the abdominal muscles don’t have the integrity to create this power.

If you find yourself in this situation where your abdominal muscles are compromised, and you are noticing doming or pendulous (uterus) belly. There are ways to treat this including, daily mindfulness for proper core contraction, belly bands, posture awareness, taping techniques, stretches, manual care, and labor positioning strategies.

Can you birth with a compromised abdominal wall? Yes.

Is it ideal? No.

And taking the time during pregnancy to focus on the integrity of your abdominal wall will not only optimize your birth it will also lay the foundation for postpartum healing.