Why do pregnant women have lower back pain?

What happens in your body during pregnancy;
•    Weight gain
•    Hormonal changes
•    Ligamentous laxity
•    Postural changes such as forward head, rounded shoulder, slouched posture, increased lower back curvature, knee hyperextension
•    Hypermobile joints
•    Decreased balance
•    Metabolic changes, heat production increases
•    Increased heart rate
•    Decreased blood pressure
•    Respiratory rate is unchanged but deeper

Hormonal changes during pregnancy results in loosening of tight connective tissues such as the ligaments, muscles.
Beside that, the big abdomen tilts the pelvis forward, increases lower back curvature, which in turn increases the pressure on soft tissues, ligaments, muscles and joints in lumbar vertebral column.