Why pregnancy changes your body's response to exercise
Pregnancy changes your whole body, not just your shape. Understanding those changes makes it easier to know what to expect when you exercise.
Your joints work differently during pregnancy
Relaxin and progesterone rise during pregnancy and loosen ligaments and tendons throughout your body, not just around the pelvis, to prepare for birth. This is the same process that allows your pelvis to open for labour. It affects joints across the whole body, not just the ones you might expect. Most women never think about their joints before pregnancy. During pregnancy they may notice them for the first time, not because something is wrong, but because ligaments and tendons that were once quietly doing their job are now a little looser, a little less supportive, and for the first time making themselves known.
Your centre of gravity shifts
Your body changes shape in ways that go well beyond the visible bump. Your centre of gravity moves forward, which changes your posture, your balance and the way you load your joints when you move. Exercises that felt unremarkable before pregnancy can feel genuinely different, and sometimes less stable, for this reason alone.
Your blood is doing more than usual
Your blood volume increases substantially during pregnancy, but not evenly. Plasma volume rises faster than your red blood cell count, which dilutes the blood. This is called the physiological anaemia of pregnancy and it is a normal part of how your body adapts. What it means practically is that your blood is not carrying oxygen as efficiently as it was before pregnancy, which is part of why everyday activities can feel harder than they used to.
Your breathing changes too
Progesterone increases how deeply and how much you breathe, often before your bump is even visible. Your growing uterus also reduces the space your lungs have to expand. Add the physiological anaemia described above, and it becomes clear why many women feel more breathless during pregnancy than they ever expected, even doing things that used to feel easy. This is not a fitness problem. It is simply your body doing more than it has ever done before.
Your pelvis works harder during pregnancy
The same ligament softening that prepares your body for birth also affects the joints of your pelvis. For some women this causes pelvic girdle pain, a condition that affects around 44% of Australian women during pregnancy. It can range from mild discomfort to significant pain that affects how you move and what activities are comfortable. Pelvic girdle pain can develop at any point in pregnancy and is difficult to predict, which is why some exercises that load the pelvis unevenly, particularly single leg work, need to be approached with care.
Your core works differently
As your uterus grows, your abdominal muscles stretch and lengthen. This is not an injury. It is a normal structural change, and it means your core functions differently than it did before pregnancy. Some movements will need to be adjusted to work with that change rather than against it.
What this means for exercise
None of these changes mean you should stop moving. Staying active during pregnancy supports your health and your baby’s. What it does mean is that some adjustments make sense, not because your body is fragile, but because it is doing something physiologically enormous.
The rest of this series looks at specific questions: lying on your back, balance and unstable equipment, running, single leg work, abdominal exercises, and weightlifting. Each one comes back to these same underlying changes.
References
Ceprnja, D., Chipchase, L., Fahey, P., Liamputtong, P., & Gupta, A. (2021). Prevalence and factors associated with pelvic girdle pain during pregnancy in Australian women: a cross-sectional study. Spine, 46(14), 944–949.
Ejikeme, C., Nandakumar, V., & Gotur, D. (2025). Respiratory physiological changes in pregnancy. Respiratory Medicine, 246, 108245. https://doi.org/10.1016/j.rmed.2025.108245
Kennedy, H., Haynes, S. L., & Shelton, C. L. (2022). Maternal body weight and estimated circulating blood volume: a review and practical nonlinear approach. British Journal of Anaesthesia, 129(5), 716–725. https://doi.org/10.1016/j.bja.2022.08.011
Soma-Pillay, P., Nelson-Piercy, C., Tolppanen, H., & Mebazaa, A. (2016). Physiological changes in pregnancy.
Cardiovascular Journal of Africa, 27(2), 89–94. https://doi.org/10.5830/CVJA-2016-021
WHEN Exercise and Pregnancy Guidelines, Version 2 (April 2026). Women’s Health Education Network. www.when.org.au
Developed by the WHEN Clinical Team
Clinically reviewed by the WHEN Clinical Governance Committee
Last reviewed: 02/07/2026
