Meet the Muscle Everyone Blames for the Postpartum Belly
Your rectus abdominis did not get lazy in pregnancy. It moved sideways. Once you understand why, a lot of confusing advice suddenly makes sense.
Quick answer
The rectus abdominis is the long, paired muscle that runs down the front of your abdomen from your lower ribs and breastbone to your pubic bone. It is the muscle people mean when they say six pack, and the horizontal bands you can sometimes see are tendinous intersections rather than separate muscles. Its main jobs are bending your trunk forward, resisting arching in your lower back, and helping control pressure inside your abdomen. In pregnancy the two halves are pushed apart because the connective tissue joining them in the middle stretches, which is what diastasis recti describes. The muscle itself is usually intact and working. That is why the answer is rarely more crunches.
Where the rectus abdominis sits and what it looks like
Picture two long straps of muscle running vertically down the front of your belly, side by side. Each strap starts up at the fifth to seventh rib cartilages and the bottom of your breastbone, and runs down to attach at your pubic bone. That is your rectus abdominis, and rectus simply means straight in Latin, which is a rare case of anatomical naming being helpful.
Two features matter for postpartum mums. The first is that the two straps do not touch. They are joined down the midline by a seam of connective tissue called the linea alba, which runs from your breastbone to your pubic bone. The second is that each strap is crossed by bands of tendon, the tendinous intersections, which is why a very lean and well trained abdomen looks segmented. The number of visible segments is largely down to genetics and body fat, not effort.
The whole muscle sits inside a wrapper called the rectus sheath, formed by the flat sheets of the muscles either side of it. That wrapper is part of why the abdominal wall behaves as one connected system rather than a set of independent parts.
muscle groups make up the abdominal wall: the rectus abdominis, the external obliques, the internal obliques and the transverse abdominis.
of women still had a separation between the two halves at six weeks postpartum in a Norwegian cohort study (Sperstad et al., British Journal of Sports Medicine, 2016).
still had one at twelve months in that same study, which puts the common fear that nothing ever changes into perspective.
What it actually does
The textbook job of the rectus abdominis is trunk flexion, which means curling your ribs towards your pelvis. That is the crunch movement, and it is the one everybody knows. But it is not the job that dominates your day as a new mum.
Far more often the muscle is working isometrically, meaning it holds without shortening. It stops your lower back arching when you carry a baby on the front. It steadies your torso while you push a pram uphill. It contributes to the container of pressure inside your abdomen every time you cough, laugh or lift the car seat. In real life it is more of a stabiliser and pressure manager than a curling muscle.
It also never works alone. The obliques rotate and side bend, and the deep transverse abdominis wraps horizontally like a corset. Underneath it all your pelvic floor forms the base and your diaphragm forms the lid. Good function is these parts timing themselves together, not any single one being strong in isolation.
The abdominal wall at a glance
| Muscle | Direction of fibres | Main job | Postpartum relevance |
|---|---|---|---|
| Rectus abdominis | Vertical, front of the abdomen | Trunk flexion and anti arching stability | The two halves separate at the midline in diastasis recti |
| External obliques | Diagonal, downward and inward | Rotation and side bending | Often stiff on one side from carrying a baby on the same hip |
| Internal obliques | Diagonal, upward and inward | Rotation and trunk control | Work with the transverse abdominis to manage pressure |
| Transverse abdominis | Horizontal, deepest layer | Tensions the midline and supports the trunk | The usual first target in postpartum core rehabilitation |
What pregnancy does to it
As your uterus grows, the abdominal wall has to make room. The rectus abdominis lengthens, and crucially the linea alba between the two halves stretches and thins. Hormonal changes in pregnancy, including relaxin, contribute to how connective tissue behaves during this period. The result is that the two straps end up further apart than they started, with a softer, wider seam between them.
This is a normal adaptation, not an injury or a failure. Almost every pregnancy produces some version of it by the third trimester. The question after birth is not whether it happened, but how well the midline regains tension over the following months, which is why our guide to checking yourself at home asks you to feel for springiness as well as width.
The one sentence that reframes everything
In most postpartum bellies the rectus abdominis is not weak or damaged, it is simply positioned wider apart with a stretched seam in the middle, so the goal is restoring tension and coordination across that midline rather than punishing a muscle that never stopped working.
Why hammering this muscle backfires
If you think the problem is a weak rectus abdominis, crunches look like the obvious answer. In practice they are often the worst early choice. Curling forward hard shortens the muscle and drives pressure forward against the very tissue that is trying to recover. That is what produces the ridge or dome you sometimes see down the middle of the belly during a sit up, which we cover in detail in coning versus doming.
The same logic explains why passive gadgets miss the point. Devices that contract the muscle for you are aimed at a muscle that was never the limiting factor, which is one of the reasons we were unconvinced by ab stimulator belts for diastasis recti. If you want the fuller argument on why the classic ab exercise is not the fix, we made it in why crunches can make things worse.
None of this means the rectus abdominis is off limits forever. It is a muscle, it responds to training, and most mums will eventually flex their trunk under load again quite happily. It is a sequencing question, not a banned list.
What to do with this information
Three practical takeaways. First, stop grading yourself on the gap width alone, because how the midline generates tension matters at least as much as how many fingers fit in it. Second, start with breathing and deep core work that builds pressure control from the inside out, before you load the rectus abdominis directly. Third, progress on how your body responds rather than on a calendar, watching for doming, heaviness or leaking as the signals to ease back.
Know the anatomy, then follow the sequence
Core Reconnect is our one time postpartum core guide with no subscription and no app to cancel. It takes you through breathing, deep core reconnection and progressive loading in short sessions designed for the gaps in a new mum's day, so you are not guessing what comes next. It is the affordable one time alternative to monthly programmes. Use code GLOW20 for 20 percent off.
Frequently asked questions
Is the rectus abdominis one muscle or two?
Anatomically it is a paired muscle, so you have a left and a right side, joined down the middle by the linea alba. Most people talk about it as one muscle because the two halves normally sit close together and act together.
Does diastasis recti mean my rectus abdominis is torn?
No. Diastasis recti describes the two halves of the muscle sitting further apart because the connective tissue between them has stretched. The muscle bellies themselves are typically intact. A tear is a different and much rarer injury.
Can I get my six pack definition back after having a baby?
Visible definition depends on body composition, genetics and training history, and it is not a fair measure of whether your core is working well. A well functioning abdominal wall that supports you without pain or leaking is the meaningful goal. Results vary from mum to mum.
How do I know if I am using my rectus abdominis instead of my deep core?
A common clue is that the belly bulges or ridges forward during an exercise rather than staying flat and drawing gently inward, and that you feel the effort near the surface. A women's health physiotherapist can assess this properly and give you feedback in person.
When can I go back to sit ups and planks?
There is no single date. Most guidance suggests building deep core control first, then reintroducing loaded flexion gradually once you can manage pressure without doming, heaviness or leaking. Speak to your GP, midwife or a women's health physiotherapist about your own timeline, especially after a caesarean.
Sources: Cleveland Clinic, rectus abdominis and diastasis recti overviews; NHS guidance on your body after childbirth; American College of Obstetricians and Gynecologists, exercise after pregnancy; Sperstad JB et al., Diastasis recti abdominis during pregnancy and 12 months after childbirth, British Journal of Sports Medicine, 2016.
This article is general education, not medical advice, and Mumma Glow is not affiliated with any programme or brand named elsewhere on this blog. Results vary from mum to mum. Please speak to your GP, midwife or a women's health physiotherapist before starting or changing what you do, particularly if you have pain, leaking, a very wide gap or a recent caesarean.