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Structurally, as your uterus accommodates for your growing fetus and you gain weight, the muscles of your abdomen (rectus abdominis) stretch and become weak. This causes your back muscles and spine to be the primary load bearer of your upper body. Thus, it is not surprising that women reporting the most intense PLBP are in their third trimester when the baby is at its largest.
As your lower abdomen moves forwards and downwards, this shifts your centre of gravity anteriorly. Although you are prevented from falling forwards during daily activities by the increased tension in your lower spine, your posture is skewed for the worse.
On top of that, during pregnancy a hormone called relaxin is released 10X more than during non-pregnancy. Relaxin is not all bad, it dilates your blood vessels thus increasing the blood volume to your fetus, which is vital to ensure sufficient nutrients is being supplied for his growth. It also relaxes the joints of your pelvic bone to accommodate the increase in size of your baby making it easier when the times comes to delivery the baby. However, your pelvis, which was previously a source of strength, now becomes loose and mobile.
Thus, distribution of weight from the upper body to the lower body becomes ineffective leaving the lower back to carry all the strain. Yikes! Talk about double trouble!
Source:
Katonis, P. et al. (2011). Pregnancy-Related Low Back Pain. 15(3). Pg. 205-210.
