Do you see that bulge right there at the Sacrum? That is called the Rhombas of Michaelis. Jean Sutton, a midwife from New Zealand described it as: This wedge-shaped area of bone moves backwards during the second stage of labor and as it moves back, it pushes the wings of the ilea out, increasing the diameters of the pelvis. We know it’s happening when the woman’s hands reach upwards (to find something to hold onto) , her head goes back and her back arches. It’s what Shelia Kitzinger was talking about when she recorded Jamaican midwives saying the baby will not be born ‘till the woman opens her back.’
I’m sure that is what they mean by the ‘opening of the back.’ The reason that the woman’s arms go up is to find something to hold onto as her pelvis is going to become destabilized. This happens as part of physiological second stage: it’s an integral part of an active normal birth. If you’re going to have a normal birth, you need to allow the Rhombus of Michaelis to move backwards to give the baby the maximum amount of space to turn his shoulders in. Although the Rhombus appears high in the pelvis and the lower lumbar spine when it moves backwards, it has the effect of opening the outlet as well.
When women are leaning forward, upright, or on their hands and knees, you will see a lump appear on their back, at and below waist level. It’s much higher up than you might think; you don’t look for it near her buttocks, you look for it near her waist. You can also feel it on the woman’s back. It’s a curved area of tissue that moves up into your hand, or you may suddenly see the mother grasp both sides of the back of her pelvis as the ilea are pushed out and she is suddenly aware of those muscles that have never been stretched before. Normally, the Rhombus is only out for a matter of minutes, it comes out just as second stage starts, and it’s gone back in again by the time that the baby’s feet are born, in fact, sometimes more quickly than that.
This image that I captured at my last birth, was the most pronounced that I had seen of this bulge. The most fascinating part was that I took this picture during early labor, the baby did not arrive for another 27 hours. I was very confused because, like Jean wrote, we usually only see this right before pushing.
Our bodies are amazing and I am continually surprised and learn something new at every birth.