Why does the stool of a pregnant woman change color after taking Matorna Calcium Tablets?

Patient's question:

No

Doctor's answer:

Taking Caltrate, the yellow urine should be because I don't drink enough water. Whenever I drink less water, my urine turns yellow, and when I drink more water, it becomes very clear. There's no black in my stool, but since getting pregnant, the color of my stool has always been a bit darker than before.