Is black diarrhea during pregnancy serious?

Iron supplements: Most likely cause is the iron supplements you are taking or the pebtobiimol you have been taking for an upset stomach. However, tell your OB and be evaluated.

Blood or medication: The most common things to make stool black are blood, usually old blood or from higher up like the stomach, or meds. Several medications or supplements can make stool black and change the quality of the stool - iron, peptobismol, some vitamins, etc. The spence or absence of othe symptoms or findings is important.

?bleeding: If it is really black and a bit shiney it may be malena. Blood leaking into the intestine becomes black in color as it passes through the bowel.Get it checked.