Your lower back or lumbar spine is made up of 5 vertebrae, intervertebral discs and various ligaments and muscles. Just below your lumbar spine is your sacrum. This bone, that looks like an upside down triangle, is part of your pelvic girdle and links your upper body to your legs. The other bones that make up the rest of your pelvis are called the ilium (or “hip” bone), ischium (or sitting bone) and pubis.
Your lumbar vertebrae and pelvis give your lower back bony stability, but what about movement?
Imagine jumping off a low wall and keeping your legs rigid and straight. When your feet hit the ground you will feel the shockwave all the way up to your ears. That’s one of the reasons your lower back needs to allow some movement – for shock absorption. Allowing movement means that your lower back needs to sacrifice some stability. The more movement there is, the less stability you will have. Unless there’s another system to make up for this decrease in stability. This is where your muscles and ligaments come in.
The effect on your Muscles and Ligaments during Pregnancy.
Firstly, muscles in and around your lower back contract (shorten) and lengthen to make movement possible. It not only makes movement possible, but it also controls the movements, keeps you from losing your balance and keeps you upright. Reaching up for something on a shelf or getting up after putting on your shoes are only possible because you have muscles in your lower back. Some of these muscles also cross your hip joint linking your upper body and your legs. If your pelvis is stable this link allows movement of your legs for walking, climbing steps, running and jumping.
Secondly, ligaments provide stability to stop too much movement of a joint. An everyday example of a joint will be something like the hinge in a car door. The hinge allows a certain amount of movement to let the door open and close. It only stops the movement if the door opens too much to prevent damage. In the same way, ligaments in your lower back allow you to bend forwards, backwards and side to side, but will stop you from going too far and straining the joints. Many of these ligaments connect your lumbar spine to your pelvis and also connect the different bones of your pelvis to each other. In contrast to your lower back, your pelvis has minimal movement because its main purpose is to support your upper body and create a stable attachment for your hip and thigh muscles.
Unfortunately, your body’s need for pelvic stability is exactly why lower back pain is so common during pregnancy.
Your body undergoes a lot of changes during pregnancy to prepare for childbirth. Hormones fluctuate, you retain water and even the shape of your body changes. These hormonal changes don’t only cause nausea, mood swings and cravings, though. One of these hormones, called relaxin, causes changes in your connective tissue specifically causing ligament laxity. This means that your ligaments allow more movement in your pelvis to eventually allow for childbirth. Before that time comes, though, this laxity also results in less stability in your lower back and pelvis that could lead to lower back pain.