Final answer:
The nurse should inform the client that black stools are a common side effect of iron supplements due to the way iron is processed in the digestive system and it's not usually a concern in the absence of other symptoms.
Step-by-step explanation:
The nurse's appropriate response to a pregnant client reporting black stools while taking iron supplements would be: "This is expected because of the way iron is broken down during digestion. Iron supplements turn a client's stools black. In the absence of cramping and abdominal pain, this is an expected finding. The client should be instructed to expect black stools." This response reassures the client that black stools are a common side effect of iron supplementation, and unless accompanied by other symptoms like abdominal pain or cramping, it is usually not a cause for concern. Iron supplements are known to cause darkened stools due to the way iron is processed in the digestive system. It is important for clients to be aware of this so that they do not become unnecessarily alarmed.