Why do pregnant women's stools turn 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, it becomes very clear. My stool isn't black, but after getting pregnant, the color of my stool seems to be a bit darker than before.