Anatomy and Physiology of Pregnancy Basics

Here’s what the research shows happens to your body during pregnancy. These are all normal, natural changes.


Your body has the amazing ability to change in order to grow a new human being inside, and you should know about what those changes are and how they can affect how you’re feeling during and after pregnancy. These changes are normal, and knowing about these changes can help us to recover optimally and prevent injuries that are common among new moms. You can read details in my book, but here are the basics:


One in five women have a permanent change in shoe size after their first pregnancy. A permanent increase, I should say. The arch in your foot is supported by ligaments and muscles that become more pliable during pregnancy because of hormonal changes. Combine that with the pregnancy weight gain and the result is a flattened arch. A flatter arch makes for a longer foot, and for many women the changes are significant enough that their feet don’t ever quite go back to their pre-pregnancy shape and size.1


Changes in the arches of your feet will result in a change in alignment at the joints above, including the ankles, hips, and knees. A flattened arch changes the angle of your knee joint just slightly, which can affect the mechanics on the inside and outside of your knee as well as the joint where your kneecap meets your femur (thigh bone).


Add that to the increased forces through your knees every time you sit down with a pregnant belly (on average, the force through your knees is 120% of normal, which can add up over time), and it’s no wonder many pregnant women will experience knee pain.2 If you have knee pain during pregnancy, strengthening your hip muscles and improving your squat mechanics (working in a pain-free range) will be key to recovery.


During pregnancy, your pelvic width increases, and you use the muscles on the outside of your hips more than your bottom muscle (gluteus maximus) to walk. The outer hip muscles, the gluteus medius in particular, get stronger during pregnancy, while your gluteus maximus gets weaker.3 That explains why you might notice a smaller behind during pregnancy. If you feel like your behind got smaller during pregnancy, it did!


After pregnancy, your normal biomechanics will return, and so will your backside (fortunately or unfortunately, however you see it). But it will take time. And during pregnancy, it’s a good idea to strengthen those outer hip muscles. If your gluteus medius muscles are weak, particularly if one side is weaker than the other, you’re more likely to experience low back pain during pregnancy.


Your pelvic width increases by 10-15% during pregnancy (don’t worry, your pelvis will return to its normal size after pregnancy, but it does take some time).4 This change in width also affects the angle of your knee joints (see above), contributing to the change in angle from your flattened arches.


The joints where your pelvis meets your spine (sacroiliac or SI joints) are inherently very strong, stable joints with just a little bit of movement to them. They get a lot of that stability from all of the muscles and ligaments that cross the SI joints. Those muscles include hip muscles as well as abdominal and pelvic floor muscles.5,6


Since your pelvic width changes affect the angle of pull for all of these muscles and your pelvic floor muscles are gradually stretching with the weight of your growing baby, those muscles work in a slightly different way, and that affects the stability of the joints they cross.


Your ligaments are also looser during pregnancy because of the increase of a hormone called relaxin 7-11 in your body, and that’s a wonderful thing because it allows your pelvis to widen to make room for baby – a side effect is that the joints those ligaments support and cross over won’t be as stable, and that includes your SI joints.


The way pregnancy affects your pelvic floor muscles gets a lot of attention because there are some pretty substantial effects. When you’re doing Kegel exercises, you’re working the muscles of your pelvic floor, a hammock-like set of muscles at the base of your pelvis that become stretched out during pregnancy.12 These are the muscles that prevent leaks or accidents and have a role in sexual function.13-15 Even after a C-section, you’re at risk for incontinence (for example, leaking when you laugh), which seems unfair, but it’s true.


Less than 1 percent of women who have never been pregnant have pelvic floor muscle-related leaks or accidents (called stress or urge incontinence). Among all new moms, 34% have stress incontinence.16 And there are lots of different kinds of pelvic floor muscle problems women experience, including difficulty getting the muscles to relax appropriately. When all pelvic floor disorders are considered, 43% of those who had a C-section have some kind of pelvic floor disorder, as do 58% of those who had a vaginal delivery.17


If you do have leaks or accidents, pain, or other symptoms in your pelvic region, please don’t be embarrassed. It’s very common, and having a pelvic floor problem can seriously affect your quality of life. Please tell your health care provider about it right away. You can find a physical therapist near you who specializes in pelvic floor disorders here.


It’s important to learn how to time the firing of those muscles, and how to turn those muscles off. Lots of women assume the only thing they need to do is strengthen the pelvic floor muscles after pregnancy, and it can be frustrating to have strong pelvic floor muscles but incontinence because the muscles are firing late or pain with sexual intercourse because the muscles are in a constant state of increased firing.


So it’s important to do all 3: re-strengthen the pelvic floor, learn timing, and learn how to relax the pelvic floor muscles. I cover the basics here and in my book, but if that doesn’t help, seeing a women’s health physical therapist (one who specializes in the pelvic floor) can make a huge difference in your life. Don’t delay. Please.

Low Back

Not surprisingly there’s a lot going on with your low back during and after pregnancy.


The common assumption is that during pregnancy the curve in the small of a pregnant woman’s back will get deeper to accommodate the center of gravity moving forward, but about half of women tilt their pelvis back to bring the weight back over their base of support (their feet).6,9,10,12 Both directions, tilting the pelvis forward or back, can add stress to the low back in different ways.


After pregnancy, tilting the pelvis back is more common – you’re still carrying your baby, just in your arms.7 That can put stress on your low back as well as your healing abdominal muscles, especially your 6-pack muscle (rectus abdominis). And especially if you’re struggling with diastasis recti, or muscle separation of the rectus abdominis. Once you’ve had your baby, watching your posture throughout the day and aiming to keep your ribs directly over your pelvis can go a long way. If you walk past a mirror while carrying your baby and observe your posture, you’ll see what I mean.


Because of these changes, it’s really important to re-strengthen your abdominal and back muscles after pregnancy as a part of your recovery.


Just like the low back, this is (obviously) affected dramatically by pregnancy. You have 4 main belly muscles that are stretched out during pregnancy.6,9,12 Your oblique muscles (internal and external) have the job of twisting and bending your spine, but they also contribute to core stability. They’re more on the sides of your belly. Right down the middle is your rectus abdominis, and its job is to bend the spine forward. Just like the obliques, it also contributes to the stability of your core.


The deepest abdominal muscle is called your transversus abdominis, and its job is to stabilize your core. It works together with the pelvic floor and your diaphragm (up next for discussion) to create a stable base for your extremities to move. Every time you move throughout the day you’ll experience a movement pattern of your core muscles turning on and off (though they’re never really 100% “off,” their “off” cycle is what we’ll call it when they’re turned down) in perfect synchrony to allow for stability as well as mobility. You can read more about movement patterns of your core here.


All of these muscles are affected during pregnancy because your belly stretches out, and so do those muscles. During pregnancy the relationship between your pelvic floor and your transversus abdominis is disrupted and must be re-trained afterwards.

Ribs, mid-back, and diaphragm

In response to the changes in angle at your low back, your mid-back will experience a change in curvature, too, that is often a more hunched posture,12 made more pronounced once your baby is born and you spend every waking moment holding that sweet little one and staring down at him or her… or several different versions of that including carrying a crying baby for hours and hours and hours trying to get baby to fall asleep. Or breast-feeding for marathon nursing sessions. All of those things can result in a hunched posture.


During pregnancy, your diaphragm shifts upwards on average 4cm (don’t worry, this also goes back to “normal” after pregnancy). To accommodate the baby and preserve your lung capacity, your rib cage also expands, on average increasing in diameter 6cm front to back.9,18 The overall result is an increased lung capacity during pregnancy, believe it or not.9,10 Along with this benefit, you’ll experience an increased VO 2 max during pregnancy. Your VO 2 is the rate at which you take in oxygen and get it to all of your muscles. You can think of it as your fuel efficiency. The VO 2 max is the maximum fuel efficiency (highway miles). If you exercise after pregnancy, guess what? You get to keep that increased VO 2 max!19


A left sided rib flare is normal (feel your ribs, and you may notice that they jut out more on the left), and during pregnancy you may experience an increase in this left sided rib flare. This will return to normal on its own after pregnancy, but it will take a long time. I haven’t seen a research study to forecast how much time this takes.


Just like changes in your low back curvature affect your mid-back, changes in your mid-back affect your neck. A more hunched mid-back posture means you’ll jut your chin out farther and rely on the tiny muscles at the base of your skull to hold your head up. When those muscles get sore, you will experience what’s known as a rams’ horn headache that goes from the base of your skull, up and over your ears, and to the sides of your forehead. Chin tucks are an important part of this program as a preventive measure and to help ease the stress on these small muscles.


Listen up. Pregnancy brain is real. During pregnancy, a study out of the UK showed that your brain shrinks significantly, on average 2-6.6%. Though the researchers were careful to point out they couldn’t draw any conclusions from these findings, we’re all thinking the same thing. But don’t worry, 6 months after you have your baby your brain goes back to normal and everything is right as rain.20

Other Changes

One of the first things that happens during early pregnancy is an increased heart rate, both at rest and while exercising.19 That’s why before you even knew you were pregnant you may have felt fatigued during your workouts, when you’re more likely to notice this effect.


You also experience a 30-40% increase in blood volume during pregnancy, and this is apparent at the 12 week mark. Since this increase in volume involves mostly an increase in blood plasma (the liquid part of the blood, as opposed to the blood cells), some women experience pregnancy-induced anemia because the overall concentration of red blood cells decreases in relation to the amount of plasma.10,19 So you may have the same number of red blood cells, but they’re “watered down” by the increased plasma, leaving you anemic. That’s why most prenatal vitamins contain iron.


I’ve heard the phenomenon of losing your pregnancy weight but not quite appearing the same as “the great shift,” but it’s nothing to be afraid of. Once you are aware of the pregnancy-related changes, you can use those changes to your advantage and recover optimally. With a targeted program this “shift” can result in your best body yet!


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2. Takeda K. A Kinesiological Analysis of the Stand-to-Sit during the Third Trimester. Journal of Physical Therapy Science. 2012;24:621-624.
3. Bewyer KJ, Bewyer DC, Messenger D. Pilot data: association between gluteus medius weakness and low back pain during pregnancy. The Iowa Orthopaedic Journal. 2009;29:97-99.
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12. Balogh A. Pilates and pregnancy. RCM Midwives. 2005;8(5):220-222.
13. Smith MD, Coppieters MW, Hodges PW. Postural activity of the pelvic floor muscles is delayed during rapid arm movements in women with stress urinary incontinence. International Urogynecology Journal. 2007;18:901-911.
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15. Junginger B, Baessler K, Sapsford R. Effect of abdominal and pelvic floor tasks on muscle activity, abdominal pressure and bladder neck. International Urogynecology Journal. 2010;21:69-77.
16. Sahakian J, Woodward S. Stress incontinence and pelvic floor excercises in pregnancy. British Journal of Nursing. 2012;21(18):S10-S15.
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