The ligament that is attached to the uterus becomes stretched, increases in diameter, and becomes very tight. The same article says when reaching or stretching this ligament can be stretched enough to cause pain in the lower part or side of the abdomen. This pain may be located in places that are out of the ordinary because organs are being pushed to different areas by the enlarging uterus. The same article recommends relaxing by either lying down, sitting in a comfortable chair, or taking a warm, soothing bath. It is advised to seek medical attention if pain is intense or if it persists.
Back pain can start to be a problem during the second trimester of pregnancy.
It says the ligaments that hold the joints in place become increasingly pliable during pregnancy. This is so the pelvis is able to widen during delivery. Because of this increased elasticity, the spine is put under a lot of strain during pregnancy. With the additional weight of the uterus, the woman's center of gravity changes. These changes can cause a lot of strain on the back, especially when standing.