Final answer:
During pregnancy, slowed gallbladder emptying due to hormonal changes can lead to the formation of gallstones by allowing bile to become overly concentrated with cholesterol.
Step-by-step explanation:
The gastrointestinal changes in pregnancy that cause gallstones are tied to how the body processes cholesterol and other substances in the gallbladder. Bile, which contains cholesterol, is stored in the gallbladder and when there is an excessive amount of cholesterol, it can precipitate out in the form of gallstones. Given the options provided, slowed gallbladder emptying (C) is the change during pregnancy that can contribute to the formation of gallstones. This happens because the hormones released during pregnancy, particularly progesterone, can relax muscles and slow down the movement of food through the gastrointestinal tract, including the gallbladder. This decreased motility means bile stays in the gallbladder longer, which can lead to gallstone formation if the bile becomes overly concentrated with cholesterol.