Final answer:
Pregnant women often experience back strain late in pregnancy due to the change in their center of gravity and hormonal changes that affect ligament and joint stability.
Step-by-step explanation:
Pregnant women often suffer from back strain late in their pregnancy due to several factors. The second and third trimesters come with significant changes in maternal anatomy, including the enlargement of the abdomen and weight gain from the growing fetus, uterus, amniotic fluid, and placenta. This additional weight shifts the center of gravity forward, leading to a change in posture to maintain balance. Consequently, muscles in the lower back counteract this with a torque that has a small mechanical advantage, requiring the muscles to exert greater force which can lead to strain.
Additionally, hormonal changes during pregnancy, specifically the release of relaxin, allow ligaments and joints in the pelvic area to relax to facilitate childbirth. However, this can also decrease spinal stability, leading to an increased risk of back strain. Therefore, changes in weight distribution and hormonal effects on the spine are the main reasons why pregnant women experience back pain, rather than due to increased muscle strength or improved posture during pregnancy.