SI Joint Pain in Pregnancy (And Why "It's Just Relaxin" Isn't the Whole Story)

If you're pregnant and something deep in the back of your pelvis, right around those two dimples above your glutes, has started aching, catching, or straight-up screaming at you when you roll over in bed or climb the stairs… you are not imagining it, and you are far from alone. Estimates suggest pelvic girdle pain affects close to half of all pregnant women, and some studies put the number even higher, with roughly eight out of ten pregnant women experiencing sacroiliac joint pain at some point. If you've been told this is just a normal, unavoidable part of pregnancy and sent on your way with nothing more than a "hang in there," I want to give you something better: what the research actually says right now.

The Old Story: "Blame Relaxin"

For years, the explanation handed to pregnant patients was tidy and hormonal: relaxin loosens your ligaments to prepare your pelvis for birth, your SI joint gets unstable, and pain follows. It's a satisfying story because it's simple, and there's truth buried in it! Hormonal shifts including relaxin and estrogen do loosen the SI joint's fibrous structures during pregnancy, increasing joint mobility.

But newer research has complicated that tidy story in an important way: when researchers actually measured relaxin levels against symptoms and physical test results, higher relaxin did not predict worse pain or disability. In other words, everyone's ligaments loosen to some degree, but that alone doesn't explain why one pregnant person sails through with mild discomfort while another is in agony climbing out of the car. Relaxin sets the stage. It isn't the whole play.

What the Research Actually Points To Now

The current thinking is more layered, and more useful, because it gives us more places to actually intervene.

It's biomechanical, not just hormonal. A 2025 review of pregnancy-related spinal biomechanics pulled together decades of literature on spinopelvic alignment and SI joint dysfunction, and the picture that emerges is one of cumulative load: a shifting center of gravity, increased lumbar lordosis, growing abdominal and intrauterine pressure, and changing posture all stacking on top of that hormonal laxity to affect spinal biomechanics and sacroiliac joint function throughout pregnancy.

Muscle activation patterns matter, maybe more than we thought. One study looking at the transverse abdominus, a key deep core stabilizer, found something counterintuitive: in people with persistent posterior pelvic girdle pain, this muscle wasn't underactive, it was firing excessively during a standard leg-raise test the transverse abdominal muscle showed excessive activity during active straight leg raising in those with pregnancy-related posterior pelvic pain. That matters clinically, because if the reflexive assumption is "just strengthen your core more," and the actual issue is a muscle that's already overworking and guarding, more bracing can make things worse, not better.

Asymmetry is a real risk factor. Not everyone's pelvis loosens evenly. Uneven laxity between the two sides of the SI joint was associated with roughly three times the risk of developing moderate to severe pelvic girdle pain, compared to people whose laxity was more symmetrical. This is part of why a hands-on assessment matters so much more than a generic exercise handout.

There's a trimester pattern. Across pregnancies, SI joint pain tends to build rather than stay flat, with research identifying the second trimester as the point where sacroiliac pain tends to peak compared to the first and third. If your pain intensified somewhere in that middle stretch and you assumed it would only keep getting worse from there, that's not necessarily true… but it is worth treating early rather than waiting.

It can show up on imaging, and it doesn't always resolve the moment you deliver. A four-year MRI follow-up study of women with low back pain found that the prevalence of sacroiliac joint findings such as bone marrow edema increased over the study period, and women whose pain began around a pregnancy showed a persistently higher prevalence of buttock pain and related findings, even though the gap between groups narrowed with time. Translation: this is a real, measurable, tissue-level phenomenon, not something you're exaggerating, and it deserves real follow-up care rather than an assumption that it will simply vanish after birth.

What Actually Helps, According to the Evidence

This is the part I care about most, because a diagnosis without a plan is just not that useful.

• Motor control and balance-based exercise, not generic core work. Research on motor control exercises, training that retrains how your deep stabilizing muscles coordinate, not just how strong they are, has shown meaningful improvements in pain and function for SI joint dysfunction, with a combined program of motor control exercises and balance training outperforming either approach alone. A separate review focused specifically on pelvic girdle pain reinforced that thoughtfully designed motor control work is one of the more promising tools we have for addressing pain localized around the sacroiliac joints in this population.

• Manual therapy, including myofascial release. Hands-on treatment continues to be studied specifically for this population, including recent trial work on myofascial release with and without a support belt for sacroiliac joint pain in pregnant patients.

• Support belts as an adjunct, not a fix on their own. Belts can offer real relief by providing external compression while your own stabilizing muscles are compromised, but the current research trend pairs them with active rehab strategies rather than treating them as a standalone solution.

• Postural retraining. A study on postpartum SI joint pain found that combining postural correction exercises with low-level laser therapy improved outcomes, another sign that this pain responds to addressing the whole system, not just the joint in isolation. (Yes, we offer this if you ask for it)

• Targeted work earlier in pregnancy, not after it's severe. Research on motor control interventions focused on spinal alignment found benefit even in populations managing additional pregnancy complications, reinforcing that a safe, non-invasive, targeted motor control approach can meaningfully reduce pregnancy-related low back and pelvic pain.

Here's What I Want You to Take From This

SI joint pain in pregnancy is common, it's real, it's measurable, and it is not simply the toll you pay for growing a human. It's also not something you have to grit your teeth through until delivery, hoping it resolves on its own. The evidence increasingly points toward a coordination and load problem as much as a hormonal one, which is genuinely good news, because coordination and load are things we can actually assess and retrain.

If you're dealing with this, you deserve more than a shrug and a belt handed to you on your way out of an appointment. You deserve someone to actually look at how your body is moving, loading, and compensating, and to build a plan around what they find.

If this sounds like you, our team offers in-home assessments built around exactly this kind of hands-on, individualized approach. You can book a consult at bewellbaby.org/book-now.

Love,

Dr. Emily

References

1. Alqhtani RS, Ahmed H, Alshahrani A, Alyami AM, Khan AR, Khan A. Synergistic Benefits of Motor Control Exercises and Balance Training in Sacroiliac Joint Dysfunction: A Randomized Controlled Trial. Life (Basel). 2023 Nov 27;13(12):2258. doi: 10.3390/life13122258. PMID: 38137859; PMCID: PMC10745022.

2. Zitti M, Mantia A, Garzonio F, Raffaele G, Storari L, Paciotti R, Fiorentino F, Andreutto R, Maselli F. Motor Control Exercises and Their Design for Short-Term Pain Modulation in Patients with Pelvic Girdle Pain: A Narrative Review. Healthcare (Basel). 2025 Mar 6;13(5):572. doi: 10.3390/healthcare13050572. PMID: 40077134; PMCID: PMC11899138.

3. Vøllestad NK, Torkildsen M, Robinson HS. Association between the serum levels of relaxin and responses to the active straight leg raise test in pregnancy. Manual Therapy. 2012;17(3):225–230. PMID not directly captured — full text: sciencedirect.com/science/article/abs/pii/S1356689X12000045

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5. Mens JMA, Pool-Goudzwaard A. The transverse abdominal muscle is excessively active during active straight leg raising in pregnancy-related posterior pelvic girdle pain: an observational study. BMC Musculoskelet Disord. 2017;18(1):372. Published 2017 Aug 25. doi:10.1186/s12891-017-1732-9

6. Żurawski AŁ, Ha SY, Świercz G, Adamczyk Gruszka O, Kiebzak WP. Targeted Motor Control Considering Sternal Position Improves Spinal Alignment in Pregnant Women at Risk for Preterm Birth with Low Back Pain.J Clin Med. 2024;13(24):7661. Published 2024 Dec 16. doi:10.3390/jcm13247661

7. Kiil RM, Arnbak BA, Zejden A, Schiøttz-Christensen B, Hendricks O, Jurik AG. Pregnancy-related sacroiliac joint findings in females with low back pain: a four-year magnetic resonance imaging follow-up study. Acta Radiol. 2022;63(6):775-784. doi:10.1177/02841851211017108

8. Yoseph ET, Taiwo R, Kiapour A, Touponse G, Massaad E, Theologitis M, Wu JY, Williamson T, Zygourakis CC. Pregnancy-Related Spinal Biomechanics: A Review of Low Back Pain and Degenerative Spine Disease. Bioengineering (Basel). 2025 Aug 10;12(8):858. doi: 10.3390/bioengineering12080858. PMID: 40868371; PMCID: PMC12383562.

9. El-Shafei MA, Abd Allah DS, Elmasry DM, Mohamed MF, Hamdy HA. Low-level laser therapy combined with postural correction exercises on postpartum sacroiliac joint pain: a randomised controlled trial. Physiother Q. 2024;32(2):92–99. doi:10.5114/pq/166400.

10. Akmal R, et al. Sacroiliac Joint Pain and its Peak Time during the Three Trimesters of Pregnancy: A Cross Sectional Study. Ann Med Health Sci Res. 2022;12:101-104.