You can think of the SI (sacroiliac) joint several ways.
Bone-wise, it’s where the sacrum links with the sides of the ilia. The sacrum looks like a huge, ancient arrowhead. It’s made from fused vertebrae, and narrows down to your coccyx (tailbone). The ilia, otherwise known as the tops of your hip bones, look like elephant ears or butterfly wings.
If you think of the forces at work when you move around, the SI joint is where the body transfers energy between the legs and upper body. So, you know, it’s pretty critical.
As you grow up and get older, the SI joint goes through several changes, which I wrote about here.
Pregnant bodies put their SI joints through even more changes, including a loosening of the strong ligaments that hold the SI joint in place. In the postpartum period, the result of this movement, or the body’s attempt to readjust back to the pre-pregnancy state, may result in muscle imbalances.
Commonly, SI joint presents as lower back pain, and/or as one-sided hip pain.