A highly durable network of ligaments surrounds these joints giving them tremendous strength. The pubic symphysis has a  fibrocartilage joint which may contain a fluid filled cavity and is avascular; it is supported by the superior and arcuate ligaments. The sacroiliac joints are synovial, but their movement is restricted throughout life and they are progressively obliterated by adhesions. The nature of the bony pelvic ring with its three joints determines that no one joint can move independently of the other two. Relaxin hormone
Relaxin is a hormone produced mainly by the corpus luteum of the ovary and breast, in both pregnant and non-pregnant females. During pregnancy it is also produced by the placenta, chorion, and decidua. The body produces relaxin during menstruation that rises to a peak within approximately 14 days of ovulation and then declines. In pregnant cycles, rather than subsiding, relaxin secretion continues to rise during the first trimester and then again in the final weeks. During pregnancy relaxin has a diverse range of effects, including the production and remodelling of collagen thus increasing the elasticity of muscles, tendons, ligaments and tissues of the birth canal in view of delivery. Although relaxins main cellular action in pregnancy is to remodel collagen by biosynthesis (thus facilitating the changes of connective tissue) it does not seem to generate musculoskeletal problems. European Research has determined that relaxin levels are not a predictor of PGP during pregnancy. Gait changes
The pregnant woman has a different pattern of "gait".
In some cases it can come on suddenly or following a fall, sudden abduction of the thighs (opening too wide too quickly) or an action that has strained the joint. PGP can begin as early as the first trimester of pregnancy. Pain is usually felt low down over the symphyseal joint, and this area may be extremely tender to the touch. Pain may also be felt in the hips, groin and lower abdomen and can radiate down the inner thighs. Women with PGP may begin to waddle or shuffle, and may be aware of an audible clicking sound coming from the pelvis. PGP can develop slowly during pregnancy, gradually gaining in severity as the pregnancy progresses. During pregnancy and postpartum, the symphyseal gap can be felt moving or straining when walking, climbing stairs or turning over in bed; these activities can be difficult or even impossible. The pain may remain static, e.g., in one place such as the front of the pelvis, producing the feeling of having been kicked; in other cases it may start in one area and move to other areas. It is also possible that a woman may experience a combination of symptoms. Any weight bearing activity has the potential of aggravating an already unstable pelvis, producing symptoms that may limit the ability of the woman to carry out many daily activities. She may experience pain involving movements such as dressing, getting in and out of the bath, rolling in bed, climbing the stairs or sexual activity.