Pelvic girdle pain is common in pregnancy. The pelvis can increase in mobility secondary to hormonal changes.  The muscles attaching to the bones of the pelvis: the sacrum, the coccyx, the hips and the lumbar spine may become stressed as they try to compensate for the increased mobility and lack of stability. They may tighten up, often developing trigger points and becoming painful.
Research has shown that asymmetry between the two sides of the pelvis i.e. hypermobility (too mobile) on one side and hypomobility (too stiff) on the other side is also one of the major causes of pain

Further research has shown that the pelvic floor muscles compensate for the increased mobility and instability of the pelvis and  also in response to increases in pain

This means that if the pelvis has become unstable during a pregnancy then the pelvic floor by way of working very hard becomes more and more taut and therefore weaker.

How to address it

Correcting pelvic alignment and symmetry (see Milltown Physiotherapy Pelvic Girdle Pain page)
Correct posture
Release pelvic floor muscle tension (Sniff Flop & Drop)
Correct pelvic floor muscle activation (Pelvic Floor Balance & Strengthening)
Improve core stability with correct alignment (Pelvic Floor and Abdominal Strengthening)

See  for more information on pelvic girdle pain related to pregnancy.