Final answer:
The least stressed muscle during pregnancy is likely to not have a direct role in pregnancy, such as muscles in the hands or feet, unlike the highly stressed myometrium.
Step-by-step explanation:
The muscle that would be the least stressed during pregnancy is likely one that is not significantly impacted by the hormonal changes and physical demands of pregnancy. Since the myometrium, the uterine smooth muscle is known to stretch significantly and become more sensitive to contractions as pregnancy progresses, it is not the least stressed muscle. This leaves us with muscles that have less direct involvement with the pregnancy process. Muscles such as those in the hands or feet could be considered less stressed compared to the muscles involved in supporting and moving the increased body weight or those that are affected by the hormonal shifts of pregnancy like the myometrium.
It is essential to understand that progesterone inhibits uterine contractions mainly in the first several months of pregnancy, with its levels plateauing and then dropping as the pregnancy progresses. Contrarily, estrogen levels continue to rise, and in the eighth month, fetal cortisol contributes to this increase, further sensitizing the myometrium to contractile stimuli as progesterone's calming effects wane.