Sacroiliac (SI) joint pain most commonly begins in the second trimester of pregnancy, around weeks 14 to 20, though some women notice it as early as the first trimester. Pain tends to start mild and gradually worsen as pregnancy progresses, with the most significant discomfort typically occurring in the third trimester. Research tracking pregnant women with SI dysfunction found that pain scores increased steadily from the first trimester through the third, along with worsening mobility limitations.
Why the Second Trimester Is the Turning Point
Your body begins releasing higher levels of a hormone called relaxin early in pregnancy, but the effects on your joints become noticeable once the physical changes of pregnancy start compounding. By the second trimester, several things happen at once: your uterus grows large enough to shift your center of gravity forward, your lower back curve deepens to compensate, and the ligaments around your pelvis loosen to prepare for delivery. That combination puts the SI joints, which sit at the back of your pelvis on either side of your spine, under significantly more stress than they normally handle.
The growing uterus also weakens the abdominal muscles that normally help stabilize your pelvis. As those muscles lose tension, your lumbar muscles and SI ligaments pick up the slack. This is why many women first notice a dull ache or stiffness in their lower back or buttocks somewhere around mid-pregnancy, even if the pregnancy itself feels physically manageable otherwise.
How Pain Typically Progresses
SI joint pain in pregnancy follows a fairly predictable pattern. In the early stages, it often shows up as occasional stiffness or a mild ache on one side of the lower back, usually triggered by specific movements like rolling over in bed, climbing stairs, or standing up from a seated position. Many women initially mistake it for general lower back pain.
As the third trimester begins, pain intensity typically ramps up. The weight gain accelerates, the forward tilt of the pelvis increases, and ligament laxity reaches its peak. By the late third trimester, the pain can become sharp or stabbing with certain movements, and some women find it difficult to walk long distances, stand on one leg, or sit for extended periods. Disability scores in studies mirror this progression, showing a steady climb from early pregnancy through the final weeks.
The pain is usually felt deep in the buttock on one or both sides, sometimes radiating into the back of the thigh. It’s distinct from sciatica, which tends to shoot down the entire leg, and from general lower back pain, which sits higher up near the waistline.
What Makes It Start Earlier for Some Women
Not everyone follows the same timeline. Several factors can push the onset of SI joint pain into the first trimester or make it more severe once it begins. Women who had SI joint problems in a previous pregnancy are more likely to experience pain earlier and more intensely the next time around. The ligaments and joint structures may not have fully recovered, making them vulnerable sooner.
A higher pre-pregnancy BMI increases the load on the SI joints from the start, meaning less additional weight gain is needed before symptoms appear. Previous lower back injuries, physically demanding jobs that involve a lot of standing or lifting, and carrying multiples can all accelerate onset. Women in their second or third pregnancies also tend to develop symptoms earlier, partly because the abdominal muscles have already been stretched and may provide less pelvic support from the beginning.
What’s Actually Happening in the Joint
The SI joints are designed to be extremely stable. They transfer the weight of your upper body into your legs and absorb shock when you walk. Unlike the knee or shoulder, they barely move, relying on thick ligaments and a rough, interlocking joint surface to stay locked in place.
During pregnancy, multiple forces work together to destabilize these joints. Hormonal changes loosen the tough ligaments that normally hold the joint surfaces tightly together. At the same time, the pelvis tilts forward as the belly grows, which changes the angle of force passing through the SI joints. The deepening curve of the lower back (called lordosis) shifts your center of gravity forward, and the SI joints bear a disproportionate share of the compensating force. Add in 25 to 35 pounds of weight gain, increased pressure inside the abdomen, and weakening core muscles, and the joints are absorbing loads they weren’t designed for in a loosened state.
What Helps During Pregnancy
The most effective strategies target the root cause: insufficient pelvic stability. A pelvic support belt worn low around the hips can physically compress the SI joints and reduce the amount of movement at the joint surface. Many women find immediate partial relief with one.
Strengthening the muscles around the pelvis helps compensate for the ligament laxity. Exercises like pelvic tilts, bridges, and gentle core activation (such as drawing the belly button toward the spine while on all fours) can improve stability without straining the joints. A physical therapist who works with pregnant patients can tailor a program to your specific stage of pregnancy and pain level.
Simple habit changes make a meaningful difference too. Avoid standing on one leg (sit down to put on pants and shoes), keep your knees together when rolling over in bed, and take stairs one step at a time. Sleeping with a pillow between your knees keeps the pelvis aligned and reduces overnight stress on the joints. Ice applied to the sore area for 15 to 20 minutes can help after a particularly painful day.
What Happens After Delivery
For most women, SI joint pain improves significantly within the first few months after delivery. Once the hormonal signals that loosen ligaments begin to fade, the joints gradually tighten and restabilize. This process typically takes around three to six months postpartum, though breastfeeding can prolong elevated relaxin levels and delay full recovery somewhat.
A smaller percentage of women, roughly 1 in 10, continue to experience SI joint pain beyond six months postpartum. This is more common after traumatic deliveries, in women who had severe pain during pregnancy, or when the joint surfaces shifted significantly. Targeted physical therapy postpartum can speed recovery and prevent chronic dysfunction from setting in. If pain persists beyond a year, it’s worth getting a specific evaluation of the SI joint rather than assuming it will resolve on its own.