Why is it common for women in the final weeks of pregnancy to experience frequent back pains?

The option stating that extra weight of the abdomen demands extra support (contraction) of the erector group of muscles is accurate because as the pregnancy progresses, the growing fetus adds significant weight to the front of the body. This increased weight shifts the center of gravity forward, leading to an adjustment in posture.

The erector spinae, a group of muscles located along the spine, are responsible for maintaining an upright posture and supporting the spine itself. As the abdomen's weight increases, the erector muscles must contract more to counterbalance the additional load and ensure stability. This extra demand can lead to muscle fatigue and strain, contributing to the frequent back pain that many women experience during the final weeks of pregnancy.

In contrast, the other options incorrectly attribute the support mechanisms to either the abdominal muscles or involve relaxation, which does not align with the physiological response needed to support the added weight. The abdominal muscles primarily play a role in stability and support, but during late pregnancy, the erector spinae are more directly involved in countering the effects of the increased weight.