Causes
Sometimes there is no obvious explanation for the cause of PGP but usually there is a combination of factors such as:

The pelvic joints moving unevenly. A change in the activity of the muscles in the pelvis, hip, abdomen, back and pelvic floor. A history of pelvic trauma. The position of the baby altering the loading stresses on the pelvic ligaments and joints. Strenuous work. Previous lower back pain. Previous pelvic girdle pain during pregnancy. Hypermobility, genetical ability to stretch joints beyond normal range. An event during the pregnancy or birth that caused injury or strain to the pelvic joints or rupture of the fibrocartilage. The occurrence of PGP is associated with twin pregnancy, first pregnancy and a higher age at first pregnancy. Mechanism
Pregnancy related Pelvic Girdle Pain (PGP) can be either specific (trauma or injury to pelvic joints or genetical i.e. connective tissue disease) and non-specific. PGP disorder is complex and multi-factorial and likely to be also represented by a series of sub-groups driven by pain varying from peripheral or central nervous system,
altered laxity/stiffness of muscles, laxity to injury of tendinous/ligamentous structures to mal-adaptive body mechanics.Pregnancy begins the physiological changes through a pattern of hormonal secretion and signal transduction thus initiating the remodelling of soft tissues, cartilage and ligaments. Over time, the ligaments could be stretched either by injury or excess strain and in turn may cause PGP. Anatomy
The pelvis is the largest bony part of the skeleton and contains three joints: the pubic symphysis, and two sacroiliac joints.
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.