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During labor, the physiological effect that occurs related to uterine contractions is that uterine muscle fibers shorten and thicken. As labor progresses, rhythmic contractions of the uterine muscles are initiated. These contractions are essential for the process of labor, as they help to dilate the cervix and push the fetus down the birth canal.

The mechanism behind this involves the action of the smooth muscle in the uterus. When these muscle fibers contract, they pull together and become shorter, which is necessary for effective pushing during labor. The thickening of the muscle fibers during contractions allows for increased strength and pressure to assist in the expulsion of the baby from the uterus.

Blood flow to the uterus may vary throughout labor but does not increase dramatically to the extent that it impacts the contractions themselves. Blood pressure may fluctuate during labor due to pain and stress but does not decrease significantly. Uterine smooth muscle does not relax during contractions; rather, it is the contraction that facilitates the labor process. Therefore, the shortening and thickening of the uterine muscle fibers is the quintessential physiological effect during labor.