One of the most powerful things you can understand about your postpartum body is this: your core does not work in isolation. It works in relationship specifically, the relationship between your ribcage and your pelvis.

This concept, often called your rib-pelvis stack, is at the heart of how your breathing mechanics, deep core function, and pelvic floor health all connect. And understanding it is not about finding a perfect posture or deciding that something is wrong with how you are moving. It is about giving yourself the knowledge to work with your body more effectively and to understand why certain things feel the way they do postpartum.

Let’s break it down.

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Your core is a pressure system, not just a set of muscles

Before we get into posture, it helps to understand what your core actually is  because most of us were taught to think of it as the muscles you feel burning during a plank. It is not.

Your core is a canister. A pressure system made up of four walls:

• The diaphragm on top
• The pelvic floor on the bottom
• The deep abdominals (transverse abdominis) wrapping around the front and sides
• The deep spinal muscles (multifidus) at the back

When these four work together, through your breath, through movement, under load, they create intra-abdominal pressure that stabilises your spine and pelvis. That pressure is what protects you, supports you and allows you to generate strength.

Here is the key part: for this system to work properly, all four walls need to be in the right relationship to each other. And that relationship is determined by your alignment.

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What alignment actually means

When we talk about alignment in the context of core function, we are really talking about the relationship between your ribcage and your pelvis, what is often called your rib-pelvis stack.

When the ribcage sits over the pelvis, the diaphragm and the pelvic floor are roughly parallel to each other, which means they can work in sync as you breathe. On the inhale, both move in the same direction. On the exhale, both respond together. The pressure system has the best conditions to do its job.

When that relationship shifts (as it naturally does during pregnancy) the diaphragm and pelvic floor are working at different angles to each other. Understanding that shift is the first step to working with it intentionally.

What commonly happens to the stack during pregnancy

Pregnancy asks a huge amount of the body over 9 months, and the body adapts remarkably well. But those adaptations, which are entirely necessary at the time, can linger postpartum and understanding them helps you make sense of how your body feels now.

Rib flare

As your baby grows and takes up space, your ribcage expands outward and often flares forward at the bottom. The lower ribs lift away from the pelvis to create more room. This is the body doing exactly what it needs to do.

After birth, many women find this position has simply stayed and the ribcage has adapted to that expanded shape which has become the new ‘normal’. When the ribs are in this position, the diaphragm sits in a more domed, shortened place. It has less room to descend fully on the inhale, which means the exhale is less complete, and the deep abdominals — which attach to the lower ribs — are working from a different length than they are designed to.

This is not something that is wrong with you. It is simply useful information about where your body currently is.

Anterior pelvic tilt

The weight and position of a growing baby also shifts the pelvis forward into an anterior tilt — where the front of the pelvis drops and the tailbone lifts. Again, this is a completely normal adaptation to pregnancy. The body is redistributing load and finding balance around a changing centre of gravity.

Postpartum, this tilt often persists. When the pelvis is in this position, the pelvic floor sits at a different angle, which changes how it responds to load and pressure. The glutes, which are so important for pelvic stability, are in a lengthened position and can feel harder to activate. The hip flexors, which have been working hard throughout pregnancy, tend to stay dominant.

None of this is permanent. But it is worth understanding, because it explains a lot about why certain exercises feel disconnected or why the pelvic floor seems unresponsive even when you are doing everything right.

When both happen together

Most postpartum women are experiencing some degree of both. A ribcage that has expanded and lifted, and a pelvis that has tilted forward. Together, these shifts mean the diaphragm and pelvic floor are working at greater angles to each other than they would be in a more stacked position.

This is the body’s intelligent response to nine months of significant change. The goal postpartum is not to undo what your body did and is instead, to understand it, which the goal of gently work towards a position that gives your core system the best environment to function and strengthen.

Why position matters before loading

This is where understanding your stack becomes really practical. When you train, whether that is core work, glute work, or strength training, the position you are in shapes how effectively your body can respond.

If the ribcage and pelvis are in a more stacked relationship, the diaphragm and pelvic floor can work together through the breath, the deep abdominals can engage from an optimal length, and the glutes have a better environment to activate. The same exercise, done with more awareness of position, can feel completely different.

This is not about achieving a perfect setup before you are allowed to train or trying to fix what isn’t broken. It is about building position awareness into your training from the start, so that as the load increases, the foundation underneath it is as strong as it can be.

Finding your stack

Developing awareness of your stack starts simply and it does not require any equipment.

Place one hand on your lower ribs and one on the front of your pelvis. Notice the relationship between the two. Take a breath in through your nose and feel what happens — do your ribs expand outward and backward, or does your chest lift and your lower back arch? On the exhale, let everything soften. Feel the ribs drop slightly and the pelvic floor gently respond.

There is no perfect position. What you are looking for is a sense of your ribcage sitting over your pelvis, and a breath that moves through the whole system rather than just the chest. That awareness is the foundation everything below is built on.

The exercises that help

Developing stack awareness is a combination of mobility work and strengthening, building the range of motion to find a more connected position, and then the strength to maintain it under load. The exercises below are organised into three tiers, each building on the last. This also gives you a sense of what progression should look like.

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Tier 1 — Finding the stack

These exercises are about awareness first. The goal is not to work hard, it is to feel the relationship between your ribcage and pelvis, understand where neutral sits for you, and begin connecting breath to position.

Standing wall reach with iso adductors

The wall gives immediate feedback. When you reach low and forward, the wall helps cue the ribs down and back, making it easier to feel what a more stacked position actually is. Adding the iso adductor squeeze — a ball or block between the knees — brings the pelvic floor and inner thighs into the conversation simultaneously. It is a simple exercise that teaches a lot about how the whole system connects.

90/90 hamstring bridge with band serratus reach

The 90/90 position, hips and knees at right angles with feet on a wall or bench, is one of the most useful teaching positions in postpartum training. It takes the lower back out of extension and makes it much easier to feel the pelvis and ribcage in relationship to each other. Although I always start people resting fully on the ground, this is a great variation. Adding a band reach during the bridge activates the serratus anterior, which helps draw the lower ribs down and in on the exhale. You are learning the stack through isometric movement rather than just holding a position.

Quadruped rockback with iso adductors

On hands and knees, rocking back towards the heels gives the spine a chance to find a neutral position without the challenge of gravity working against you in the same way. Adding the iso adductor squeeze again keeps the adductors engaged throughout and allows you to feel the glutes lengthening and any change in pelvis positioning. This position also makes it easier to feel posterior rib expansion on the inhale, breathing into the back of the ribcage, which is one of the key cues for restoring full breathing mechanics postpartum.

Pelvic tilt with rib block

This is as foundational as it gets. Because the bench physically blocks the ribcage from participating, all movement is isolated to the pelvis. You can feel exactly where the pelvis needs to sit to be in alignment with the fixed ribs above it. It’s common to see movement through the thoracic spine instead of pelvis so this will give you both immediate and honest feedback.You also want to be able to find a ‘neutral’ position without feeling like you are clenching your glutes to do it!

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Tier 2 — Holding the stack under demand

Once you can find the stack, the next step is holding it when something is challenging it. These exercises introduce stability demand, the body has to resist movement rather than simply finding a position.

Tall kneeling pallof press

Kneeling removes the base of support the floor gives your legs, which immediately increases the demand on the trunk. The pallof press adds a rotational challenge — the band or cable is pulling you sideways, and your job is to resist it while maintaining your stack. The key teaching point here is that resisting rotation requires the ribcage and pelvis to stay in relationship. If the ribs flare or the pelvis tilts, the resistance wins. This exercise makes the stack feel functional in a way that floor work cannot.

Short lever side plank with reach

The side plank demonstrates that rib-pelvis stacking is not just a front-to-back concept — it applies laterally too. In this position, the reach drives the top side ribs down towards the pelvis, which is a direct expression of the stack under a lateral stability demand. The short lever variation, knees bent rather than full leg extension, makes it accessible while still creating a meaningful challenge. It is also one of the most visually clear exercises for understanding what the stack looks like from a different angle.

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Tier 3 — Maintaining the stack through movement and load

The final tier is about taking everything learned in the first two tiers and applying it while the body is actively producing force or moving dynamically. This is where the stack stops being something you find and becomes something you maintain.

90/90 with iso adductors + kettlebell pullover

Back in the 90/90 position, but now with a kettlebell. The pullover, lowering the weight overhead and returning, creates a significant challenge to rib position. The natural tendency is for the ribs to flare and the lower back to arch as the arms move overhead. Your job is to use the breath and the iso adductor connection to keep the stack intact throughout the movement. This is one of the clearest ways to feel what rib control under load actually means.

Tall kneeling overhead press with isometric pulldown

The overhead press is one of the most demanding tests of rib-pelvis stack because the arms moving overhead creates a strong pull towards rib flare. Adding an isometric pulldown — actively pulling a band or cable down while pressing up — is a way of cueing the lats and lower ribcage to stay connected throughout the press. It teaches the body to produce overhead force without losing the stack, which is exactly the skill needed as training progresses into more demanding pressing patterns.

The bottom line

Understanding the relationship between your ribcage and pelvis is one of the most valuable things you can take into your postpartum training. It is not about achieving a perfect position or labelling anything as wrong. Instead, it’s knowing how your core system works, understanding how pregnancy naturally shifts things and giving your body the awareness and tools to function well under load.

Find your stack. Breathe into it. Build from there.

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If you want to start with the foundations, the assess period of Return to Strength includes a full rib flare self-assessment, a find-your-stack tutorial, and five workouts you can begin as early as two weeks postpartum. [Join here →]