Thank evolution for the relative ease with which you carry your baby.
Pregnancy requires backbone, and here's proof: Harvard research found an evolutionary adaptation to the female spine that makes it easier for pregnant women to wield their front-heavy loads. More weight is borne on joints in the lower back, which the study found are proportionally larger in women. This shift is aided by a female-exclusive exaggeration of the spine's S-curve. Nevertheless, many moms-to-be report discomfort in the lower back.
Is more relief in sight? Well, these things take time. The researchers estimate that the evolutionary shift occurred when our human precursors first walked upright—more than 2 million years ago.